drm/i915: Use intel_uncore_rmw in intel_gt_init_swizzling

Two easy opportunities to compact the code by using the existing helper.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Suggested-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621070811.7006-8-tvrtko.ursulin@linux.intel.com