drm/vmwgfx: Perform memory validations only when need full modeset.

For cases when full modeset is not requested like page-flip, skip
memory validation as the topology is not changed.

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>