drm/sti: fix warning in build.

Signed-off-by: Dave Airlie <[e-mail]>

drivers/gpu/drm/sti/sti_compositor.c

···
 						(1 << crtc) - 1, plane_type);
 				plane++;
 				break;
+			case STI_BCK:
+				break;
 			}
 
 			/* The first planes are reserved for primary planes*/

drivers/gpu/drm/sti/sti_drm_crtc.c

···
 				      struct drm_framebuffer *old_fb)
 {
 	struct sti_mixer *mixer = to_sti_mixer(crtc);
-	struct device *dev = mixer->dev;
-	struct sti_compositor *compo = dev_get_drvdata(dev);
 	struct sti_layer *layer;
 	unsigned int w, h;
 	int ret;