HW has error checks in place which check that pixel depth is explicitly
provided on DP, while HDMI has a "default" setting that we use.

In multi-display configurations with identical modelines, but different
protocols (HDMI + DP, in this case), it was possible for the DP head to
get swapped to the head which previously drove the HDMI output, without
updating HeadSetControlOutputResource(), triggering the error check and
hanging the core update.
Reported-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>