drm/i915/skl: Support the advertized index format

We advertize C8 in the primary plane formats didn't have the
corresponding code to set PLANE_CTL accordingly.
Signed-off-by: NDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -2956,6 +2956,9 @@ u32 skl_plane_ctl_format(uint32_t pixel_format)
 	u32 format = 0;
 
 	switch (pixel_format) {
+	case DRM_FORMAT_C8:
+		format = PLANE_CTL_FORMAT_INDEXED;
+		break;
 	case DRM_FORMAT_RGB565:
 		format = PLANE_CTL_FORMAT_RGB_565;
 		break;