drm/i915: Nuke display error state

drm/i915: Nuke display error state

I doubt anyone has used the display error state since CS flips
went the way of the dodo. Just nuke it.

It might be semi interesting to have something like this for
FIFO underruns and the like, but as it stands this wouldn't
provide a sufficient amount of information. So would need
an extensive rewrite anyway.

The lockless power well handling is also racy, so this could
just be contributing noise to test results if we end up
accessing something with the relevant power well already
disabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210505191140.14215-1-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>