Layering cleanups, object cleanups, kernel doc, context persistence, expose all engines to user, implement engine query uAPI, logical mapping, virtual engines, and parallel engines

Submission should be pretty rock solid after this MR. Backward breaking uAPI changes.

Tested with: https://gitlab.freedesktop.org/drm/xe/igt-gpu-tools/-/merge_requests/1/

for i in {1..4}; do xe_mmap& xe_exec_basic& xe_vm& done