drm/i915/guc: Use wait_for_register_fw() while waiting for MMIO response

drm/i915/guc: Use wait_for_register_fw() while waiting for MMIO response

Waiting for the response status in scratch register can be done
using our generic function. Let's use it.

v2: rebased

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170407160145.181328-2-michal.wajdeczko@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>