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: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 49d7227..dadb52a 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -2956,6 +2956,9 @@
 	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;