drm: don't poll forced connectors

Otherwise if the detect callback reports a different state than what
the user forced (rather likely), we continously annoy userspace about
a hotplug uevent.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>