Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: arm):

drivers/gpu/drm/sti/sti_hdmi.c: In function ‘hdmi_audio_configure’:
drivers/gpu/drm/sti/sti_hdmi.c:851:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
   audio_cfg |= HDMI_AUD_CFG_CH78_VALID;
drivers/gpu/drm/sti/sti_hdmi.c:852:2: note: here
  case 6:
  ^~~~
drivers/gpu/drm/sti/sti_hdmi.c:853:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
   audio_cfg |= HDMI_AUD_CFG_CH56_VALID;
drivers/gpu/drm/sti/sti_hdmi.c:854:2: note: here
  case 4:
  ^~~~
drivers/gpu/drm/sti/sti_hdmi.c:855:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
   audio_cfg |= HDMI_AUD_CFG_CH34_VALID | HDMI_AUD_CFG_8CH;
drivers/gpu/drm/sti/sti_hdmi.c:856:2: note: here
  case 2:
  ^~~~
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190729222752.GA20277@embeddedor