Cleanup xe_lrc

While debugging i915 I had a hard time understanding what each part of it was doing. Let's make life in xe a little bit better by cleaning up support for old platforms we don't bind xe to and documenting some pieces. The documentation part in set_offsets() I also intend to send to i915 upstream eventually.

Signed-off-by: Lucas De Marchi lucas.demarchi@intel.com