drm/mode: reduce lock hold in addfb2

No need to hold the lock while assigning the variable.

Daniel wrote:
"Not sure why exactly I put that under the lock, but the only thing that
can race here is rmfb while addfb2 is still doing it's thing, with a
correctly guess (easy to do since they're fully deterministic) fb_id."

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>