vmwgfx: Break out dirty submission code

In preperation for screen objects, still leaves the delayed workqueue
for surface updates in place.

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