drm/i915/huc: Use helper function while waiting for DMA completion

Waiting for DMA status register can be done with dedicated function.
Lets use it as additional bonus will be smaller driver footprint.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171024105056.43276-1-michal.wajdeczko@intel.com