drm/xe: Let's return last value read on xe_mmio_wait32.

This is already useful because it avoids some extra reads
where registers might have changed after the timeout decision.

But also, it will be important to end the kill of i915's wait_for.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>