drm/ttm: Add a swap_notify callback.

This is needed for a bugfix in the vmwgfx driver.
Drivers may have GPU bindings on buffers that core TTM is not aware of,
and TTM may view those buffers as ordinary system memory buffers.
Add a notifier to such drivers when TTM is about to move the buffer
contents out to swappable memory. The driver must then release any
private GPU bindings on those buffers.

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