drm/vmwgfx: add a move callback.

This just copies the fallback to vmwgfx, I'm going to iterate on this
a bit until it's not the same as the fallback path.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201006000644.1005758-4-airlied@gmail.com