drm/i915/guc: Proper xarray usage for contexts_lookup

Lock the xarray and take ref to the context if needed.

v2:
 (Checkpatch)
  - Add new line after declaration
 (Daniel Vetter)
  - Correct put / get accounting in xa_for_loops
v3:
 (Checkpatch)
  - Extra new line

Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210909164744.31249-19-matthew.brost@intel.com