drm/i915/overlay: Introduce i915_overlay_obj_lookup()

drm/i915/overlay: Introduce i915_overlay_obj_lookup()

Extract the BO lookup and tiling check into a new
helper called i915_overlay_obj_lookup(). This will have to
move to the i915 side of the parent vs. display driver split.

There is a slight change here in that we now do the tiling
check before taking the modeset locks, but those locks don't
protect the BO tiling stuff in any way, so nothing is really
different here.

Note that the hardware should support X-tiled scanout also
for the overlay, but I guess no one ever bothered to hook
it up and test it. So the check should stay at least for now.

v2: Correctly handle the ERR_PTR returned by
    i915_overlay_obj_lookup() (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260226100738.29997-7-ville.syrjala@linux.intel.com