drm/i915: add display sub-struct to drm_i915_private

In another long-overdue cleanup, add a display sub-struct to
drm_i915_private, and start moving display related members there. Start
with display funcs that need a rename anyway to not collide with the new
display member.

Add a new header under display/ for defining struct intel_display.

Rename struct drm_i915_display_funcs to intel_display_funcs while at it.

v2:
- Fix multi-line comment style (Arun)
- Use display as the member name

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/d44cae096b664f7015f8c797d6dfd4964226d4f8.1661346845.git.jani.nikula@intel.com