drm/i915: Rename PLANE_CUS_CTL Y plane bits

Rename the PLANE_CUS_CTL Y plane selection bits to actually
say "Y plane".

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211201152552.7821-6-ville.syrjala@linux.intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>