drm/i915/display: add I915 conditional build to g4x_dp.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/ba92fc1dee1f71018179dbe10b91982c9c93e7e6.1694514689.git.jani.nikula@intel.com

drm/i915/display: reduce includes in g4x_dp.h includes

Only include what's needed.

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/e68c71ad1c1c1de7f5bc659edeb208818cddde72.1667383630.git.jani.nikula@intel.com

drm/i915: Give g4x_{dp,hdmi}.c g4x_ namespace

s/intel_/g4x_/ for the externally visible g4x_{dp,hdmi}.c
functions.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210318161015.22070-8-ville.syrjala@linux.intel.com

drm/i915: Introduce g4x_dp.c

Move the g4x+ DP code into a new file. This will leave mostly
platform agnostic code in intel_dp.c. Well, the misplaced phy
test stuff pretty much ruins that, but let's squint real hard
for now.

v2: Add comment exlaining which platforms are covered (Daniel)
Leave intel_dp_unused_lane_mask() be since it is pretty generic

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210318161015.22070-6-ville.syrjala@linux.intel.com