drm/i915: add per-ring fault reg to error_state

This was pretty handy when figuring out what exactly went wrong with
ppgtt and it might also be useful when we stop filling the entire gart
with scratch page entries.

Also add the gen6+ DONE reg while at it.

v2: Chris Wilson suggested to allocate the error_state with kzalloc
for better paranoia. Also kill existing spurious clears of the
error_state while at it.

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>