drm/i915/gvt: Mark expected switch fall-through in handle_g2v_notification

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

Addresses-Coverity-ID: 1466154 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>