drm/i915: add error_state sysfs entry

As getting error state doesn't anymore require big kmallocs,
make error state accessible also from sysfs.

v2: - error state clearing (Chris Wilson)
    - user hint, proper access mode bits and name (Daniel Vetter)

v3: release resources in proper order (Chris Wilson)

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Apply Chris' s/error_state/error/ bikeshed on the sysfs
name. Also update the dmesg message, spotted by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>