drm/i915/gt: Replace I915_READ with intel_uncore_read

Get rid of the last remaining I915_READ in gt/ and make gt-land
the first I915_READ-free happy island.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
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/20191205164422.727968-1-chris@chris-wilson.co.uk