drm/i915: avoid unclaimed registers when capturing the error state

We're iterating over the CPU transcoders, so check for the correct
power domain.

This fixes many "unclaimed register" error messages.

This can be reproduced by the IGT test mentioned below, but we still
get a FAIL when we run it.

Testcase: igt/kms_lip/flip-vs-panning-vs-hang
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>