drm/xe/guc: Fix stack_depot usage

drm/xe/guc: Fix stack_depot usage

Add missing stack_depot_init() call when CONFIG_DRM_XE_DEBUG_GUC is
enabled to fix the following call stack:

	[] BUG: kernel NULL pointer dereference, address: 0000000000000000
	[] Workqueue:  drm_sched_run_job_work [gpu_sched]
	[] RIP: 0010:stack_depot_save_flags+0x172/0x870
	[] Call Trace:
	[]  <TASK>
	[]  fast_req_track+0x58/0xb0 [xe]

Fixes: 16b7e65d299d ("drm/xe/guc: Track FAST_REQ H2Gs to report where errors came from")
Tested-by: Sagar Ghuge <sagar.ghuge@intel.com>
Cc: stable@vger.kernel.org # v6.17+
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://patch.msgid.link/20251118-fix-debug-guc-v1-1-9f780c6bedf8@intel.com
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
(cherry picked from commit 64fdf496a6929a0a194387d2bb5efaf5da2b542f)
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>