drm: disable encoders before re-routing them

In some cases we may receive a mode config that has a different
CRTC<->encoder map that the current configuration.  In that case, we
need to disable any re-routed encoders before setting the mode,
otherwise they may not pick up the new CRTC (if the output types are
incompatible for example).

Tested-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>