vmwgfx: Infrastructure for explicit placement

Make it possible to use explicit placement
(although not hooked up with a user-space interface yet)
and relax the single framebuffer limit to only apply to implicit placement.

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