drm/xe/vm: Simplify and document xe_vm_lock()

drm/xe/vm: Simplify and document xe_vm_lock()

The xe_vm_lock() function was unnecessarily using ttm_eu_reserve_buffers().
Simplify and document the interface.

v4:
- Improve on xe_vm_lock() documentation (Matthew Brost)
v5:
- Rebase conflict.

Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230908091716.36984-3-thomas.hellstrom@linux.intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>