drm/i915: Extract engine fault reset to a helper

Just tidying the flow a bit.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190607082557.31670-4-tvrtko.ursulin@linux.intel.com