drm/i915/display: add I915 conditional build to i9xx_wm.h

Add stubs for !I915.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/bf15b14d3d060fdb256c78d4b8514d642a8958e6.1694514689.git.jani.nikula@intel.com

drm/i915/wm: remove stale and unused ilk_wm_max_level() declaration

The declaration was removed earlier, but got accidentally resurrected in
i915xx_wm.[ch] refactoring. Remove harder.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230419115449.422214-1-jani.nikula@intel.com

drm/i915/wm: move ILK watermark sanitization to i9xx_wm.[ch]

Move sanitize_watermarks() to i9xx_wm.[ch] and rename as
ilk_wm_sanitize(). The slightly unfortunate downside is having to expose
intel_atomic_check() from intel_display.c, but this declutters
intel_display.c nicely.

v2:
- Move to i9xx_wm.[ch] instead of intel_wm.[ch] (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215141910.433043-1-jani.nikula@intel.com

drm/i915/wm: add .get_hw_state to watermark funcs

Get rid of the if ladder in intel_modeset_setup_hw_state() and hide a
number of functions by adding a .get_hw_state() hook to watermark
functions. At least for now, combine the platform specific sanitization
to the hw state readouts on the relevant platforms instead of adding a
separate hook for that.

There's a functional change on PCH split platforms: If i9xx_wm_init()
fails to read plane latency and chooses the nop functions,
ilk_wm_get_hw_state() won't get called for readout. Add the
ilk_init_lp_watermarks() call on that path which now won't be called in
.get_hw_state(), as it looks like the only thing that could make a
difference.

v2:
- Add missing static (kernel test robot)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6da32831e40606cc8b90491b83196917f2ce36ab.1676317696.git.jani.nikula@intel.com

drm/i915/wm: move remaining watermark code out of intel_pm.c

Add new files intel_wm.[ch] and i9xx_wm.[ch] under display/ to hold
generic and pre-SKL watermark code, respectively. SKL+ watermark code
has already been split out to skl_watermark.[ch].

Use the _wm.[ch] naming for brevity; we may want to rename
skl_watermark.[ch] later accordingly.

Add new intel_wm_init() to call either skl_wm_init() or
i9xx_wm_init(i915) depending on the platform, the latter comprising of
the remains of intel_init_pm().

Sprinkle in some minor checkpatch fixes while moving the code.

v2:
- Rebase
- Fix copyright year

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/ddf04a07a37f0368b3fef85d4ebb924082fec6cd.1676317696.git.jani.nikula@intel.com