The fb already contains a DRM fd for later use. So just use that one instead of
fetching it from the backend.
This is necessary if the fbs are allocated on different devices, since otherwise
the wrong device might be used to get the fd of the passed fb.
Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>