vmwgfx: Fix fb VRAM pinning failure due to fragmentation

If the soon-to-be scanout buffer is partly covering the intended
VRAM region, move and pin will fail. In that case, just move it out
to system before attempting to move it in again.

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