drm/via: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

This patch fixes the following warnings:

drivers/gpu/drm/via/via_dmablit.c:179:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/gpu/drm/via/via_dmablit.c:185:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/gpu/drm/via/via_dmablit.c:187:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/gpu/drm/via/via_dmablit.c:195:3: warning: this statement may fall through [-Wimplicit-fallthrough=]

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enabling
-Wimplicit-fallthrough.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190129201742.GA25660@embeddedor