drm/i915: add power well and cpu transcoder info to the error state

We need to dump these registers if we want to properly interpret the
others.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>