vmwgfx: Add vblank stubs

This fixes kernel panics when running the vbltest from the drm repo. We
can't just skip initializing the vblank system since it sets up certain
state for us, see: "vmwgfx: Enable use of the vblank system."

Signed-off-by: Jakob Bornecrantz <jakob@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>