drm/radeon/kms: get HPD info for connectors

This populates the connectors with HPD (Hot Plug Detect)
information.  This will be used in subsequent patches
for automatic digital monitor connect/disconnect handling.

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