drm/sun4i: tcon: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: sunxi_defconfig arm):

drivers/gpu/drm/sun4i/sun4i_tcon.c: In function ‘sun4i_tcon0_mode_set_dithering’:
drivers/gpu/drm/sun4i/sun4i_tcon.c:318:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
   val |= SUN4I_TCON0_FRM_CTL_MODE_B;
drivers/gpu/drm/sun4i/sun4i_tcon.c:319:2: note: here
  case MEDIA_BUS_FMT_RGB666_1X18:
  ^~~~
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>

drivers/gpu/drm/sun4i/sun4i_tcon.c

@@ -314,6 +314,7 @@ static void sun4i_tcon0_mode_set_dithering(struct sun4i_tcon *tcon,
 		/* R and B components are only 5 bits deep */
 		val |= SUN4I_TCON0_FRM_CTL_MODE_R;
 		val |= SUN4I_TCON0_FRM_CTL_MODE_B;
+		/* Fall through */
 	case MEDIA_BUS_FMT_RGB666_1X18:
 	case MEDIA_BUS_FMT_RGB666_1X7X3_SPWG:
 		/* Fall through: enable dithering */