drm/i915: Augment i915 error state to include the dump of GuC log buffer

Added the dump of GuC log buffer to i915 error state, as the contents of
GuC log buffer would also be useful to determine that why the GPU reset
was triggered.

v2:
- For uniformity use existing helper function print_error_obj() to
  dump out contents of GuC log buffer, pretty printing is better left
  to userspace. (Chris)
- Skip the dumping of GuC log buffer when logging is disabled as it
  won't be of any use.
- Rebase.

v3: Rebase.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Akash Goel <akash.goel@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>