drm/radeon/kms: set HPD polarity in hpd_init()

Polarity needs to be set accordingly to connector status (connected
or disconnected). Set it up in hpd_init() so first hotplug works
reliably no matter what is the initial set of connector. hpd_init()
also covers resume so HPD will work correctly after resume as well.

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