drm/xe: Rename GPU offset helper to reflect true usage

The _io_offset helper function is returning an offset into the GPU
address space.  Using the CPU address offset (io_) is not correct.

Rename to reflect usage.
Update to use GPU offset information.

Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>