drm/xe/bo: Simplify xe_bo_lock()

drm/xe/bo: Simplify xe_bo_lock()

xe_bo_lock() was, although it only grabbed a single lock, unnecessarily
using ttm_eu_reserve_buffers(). Simplify and document the interface.

v2:
- Update also the xe_display subsystem.
v4:
- Reinstate a lost dma_resv_reserve_fences().
- Improve on xe_bo_lock() documentation (Matthew Brost)

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/20230908091716.36984-2-thomas.hellstrom@linux.intel.com
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>