drm/radeon/kms: fix regression in hotplug

Skip connectors that do not have an HPD pin.

Should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=39027



Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Tested-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>