vmwgfx: Place overlays in GMR area if we can

When we hae screen objects we are allowed to place the overlay source
in the GMR area, do this as this will save precious VRAM.

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