drm/radeon: Unbreak HPD handling for r600+

drm/radeon: Unbreak HPD handling for r600+

commit 3d18e33735a02b1a90aecf14410bf3edbfd4d3dc upstream.

We end up reading the interrupt register for HPD5, and then writing it
to HPD6 which on systems without anything using HPD5 results in
permanently disabling hotplug on one of the display outputs after the
first time we acknowledge a hotplug interrupt from the GPU.

This code is really bad. But for now, let's just fix this. I will
hopefully have a large patch series to refactor all of this soon.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lyude <lyude@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.16: drop the DC_HPD6_RX_INTERRUPT cases]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>