drm/msm: Annotate intentional switch statement fall throughs

Explicitly mark intentional fall throughs in switch statements to keep
-Wimplicit-fallthrough from complaining.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1564073588-27386-1-git-send-email-jcrouse@codeaurora.org