drm/sun4i: sun6i_mipi_dsi: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: multi_v7_defconfig arm):

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c: In function ‘sun6i_dsi_transfer’:
drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c:993:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (msg->rx_len == 1) {
      ^
drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c:998:2: note: here
  default:
  ^~~~~~~

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>