drm/xe/display: rename xe_plane_initial.c to xe_initial_plane.c
Follow i915 with the more naturally flowing naming.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/62eb56fe348a8fe7c17333d784192da701367cc7.1765812266.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>