drm/i915: Make i915_destroy_error_state take dev_priv

Since it does not need dev at all.

Also change the stored pointer in struct i915_error_state_file_priv
to i915.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>