drm/i915/overlay: Convert overlay to parent interface

drm/i915/overlay: Convert overlay to parent interface

Convert the direct i915_overlay_*() calls from the display
side to go over a new parent interface instead.

v2: Correctly handle the ERR_PTR returned by
    i915_overlay_obj_lookup() (Jani)
v3: Rebase due to the NULL check in intel_overlay_cleanup()

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260226130150.16816-1-ville.syrjala@linux.intel.com