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

Even though we only check for unclaimed registers while we're writing
registers, if we read a bad register we'll still trigger a CPU error
interrupt, and we'll print an "Unclaimed register" DRM_ERROR due to
that. To avoid this error, just avoid touching power domains that are
not enabled.

Use kzalloc so we're sure all the disabled domains will be zeroed on
the error state file. We already print the information that is enough
to discover if the power well is enabled on the error state file, so
this should not be a problem.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=69747


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