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>