drm/i915/gt: Replace I915_WRITE with its uncore counterpart

Get rid of the last remaining I915_WRITEs and replace them with
intel_uncore_write().

Signed-off-by: Andi Shyti <andi.shyti@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/20191206212417.20178-1-andi@etezian.org