drm/i915: Convert PAT setup to uncore mmio

One more thing which relied on implicit dev_priv can be covnerted to use
the new mmio accessors.

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/20191024093440.32280-1-tvrtko.ursulin@linux.intel.com