drm: Check for a NULL encoder when reverting on error path

We need to skip the connectors with a NULL encoder to match the success
path and avoid an OOPS.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>