drm/dp: annotate implicit fall throughs

There is a plan to build the kernel with -Wimplicit-fallthrough and
these places in the code produced warnings (W=1). Fix them up.

This commit remove the following warnings:

  include/linux/compiler.h:77:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
  include/asm-generic/bug.h:134:2: note: in expansion of macro 'unlikely'
  drivers/gpu/drm/drm_dp_helper.c:155:3: note: in expansion of macro 'WARN'
  include/linux/compiler.h:77:22: warning: this statement may fall through [-Wimplicit-fallthrough=]
  include/asm-generic/bug.h:134:2: note: in expansion of macro 'unlikely'
  drivers/gpu/drm/drm_dp_helper.c:173:3: note: in expansion of macro 'WARN'
  drivers/gpu/drm/drm_dp_helper.c:547:3: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Acked-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190114202748.15584-1-malat@debian.org