drm/vmwgfx: move null mem checks outside move notifies

Both fns checked mem == NULL, just move the check outside.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201006000644.1005758-3-airlied@gmail.com