drm/xe/oa: Fix static checker warning about null gt

There is a static checker warning that gt returned by xe_device_get_gt can
be NULL and that is being dereferenced. Use xe_root_mmio_gt instead, which
is equivalent and cannot return a NULL gt 0.

Fixes: 10d42ef ("drm/xe/oa: Assign hwe for OAM_SAG")
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://lore.kernel.org/r/20250715181422.2807624-1-ashutosh.dixit@intel.com

Author: ashutoshx

drivers/gpu/drm/xe/xe_oa.c

@@ -1941,7 +1941,7 @@ static int xe_oa_assign_hwe(struct xe_oa *oa, struct xe_oa_open_param *param)
 
 	/* If not provided, OA unit defaults to OA unit 0 as per uapi */
 	if (!param->oa_unit)
-		param->oa_unit = &xe_device_get_gt(oa->xe, 0)->oa.oa_unit[0];
+		param->oa_unit = &xe_root_mmio_gt(oa->xe)->oa.oa_unit[0];
 
 	/* When we have an exec_q, get hwe from the exec_q */
 	if (param->exec_q) {