drm/i915: Convert some more bits to use engine mmio accessors

Remove a couple dev_priv locals as a consequence.

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/20190607084521.16845-1-tvrtko.ursulin@linux.intel.com