drm: Only decouple the old_fb from the crtc is we call mode_set*

Otherwise when disabling the output we switch to the new fb (which is
likely NULL) and skip the call to mode_set -- leaking driver private
state on the old_fb.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=29857
Reported-by: Sitsofe Wheeler <sitsofe@yahoo.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>