drm: Make the CRTC gamma_set operation optional

Drivers for hardware without gamma support should not be forced to
implement a no-op gamma set operation.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Rob Clark <rob.clark@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index 404597e..6cb47d9 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -3332,6 +3332,11 @@
 	}
 	crtc = obj_to_crtc(obj);
 
+	if (crtc->funcs->gamma_set == NULL) {
+		ret = -ENOSYS;
+		goto out;
+	}
+
 	/* memcpy into gamma store */
 	if (crtc_lut->gamma_size != crtc->gamma_size) {
 		ret = -EINVAL;