drm/radeon: Unbreak HPD handling for r600+

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>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>