drm/i915: Expose C8 on VLV/CHV sprite planes

VLV/CHV sprite planes also support the C8 format. Let's expose that.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191031165652.10868-5-ville.syrjala@linux.intel.com