drm/radeon: set hpd polarity at init time so hotplug detect works

Polarity needs to be set accordingly to connector status (connected
or disconnected). Set it up at module init so first hotplug works
reliably no matter what is the initial set of connector.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>