drm/radeon/kms: don't enable connectors that are off in the hotplug handler

If we get a hotplug event on an connector that is off, don't
attempt to turn it on or off, it should already be off.

Fixes:
https://bugzilla.redhat.com/show_bug.cgi?id=728228



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>