drm/i915: Rename i915_{save,restore}_state()

i915_{save,restore}_state() are actually all about the display.
Currently they are split into display part + SWF part. But since
the SWF part is also related to the display let's just move that
part into its own thing and flip the roles around so that the
current display part is the main function.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201005171441.26612-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>

drm/i915: extract i915_suspend.h from i915_drv.h

It used to be handy that we only had a couple of headers, but over time
i915_drv.h has become unwieldy. Extract declarations to a separate
header file corresponding to the implementation module, clarifying the
modularity of the driver.

Ensure the new header is self-contained, and do so with minimal further
includes, using forward declarations as needed. Include the new header
only where needed, and sort the modified include directives while at it
and as needed.

No functional changes.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/94f2884a3e5611c3e1f015104afb965e47bd8992.1565271681.git.jani.nikula@intel.com