drm/i915: Allow horizontal mirroring for cnl+ "sprite" planes

All CNL universal planes support horizontal mirroring. Currently
we expose the capability only for the primary plane. Expose it
for the overlay planes as well.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181005125817.22576-5-ville.syrjala@linux.intel.com