drm/i915/display: convert skl_universal_plane.c to struct drm_gem_object

drm/i915/display: convert skl_universal_plane.c to struct drm_gem_object

Prefer the driver agnostic struct drm_gem_object over i915 specific
struct drm_i915_gem_object. Add new intel_bo_* functions as
needed. Convert intel_pxp_key_check() to struct drm_gem_object.

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0a6d2bec50764efaae4322c9cfa33eefbfe1c054.1726589119.git.jani.nikula@intel.com