drm: don't unlock in the addfb error paths

We don't grab the modeset locks any more since

commit 468174f7
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Tue Dec 11 00:09:12 2012 +0100

    drm: push modeset_lock_all into ->fb_create driver callbacks

Reported-by: Ray Strode <rstrode@redhat.com>
Cc: Ray Strode <rstrode@redhat.com>
Cc: Dave Airlie <airlied@gmail.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>