drm/xe: Grab mem_access when disabling C6 on skip_guc_pc platforms

If skip_guc_pc is set for a platform, C6 is disabled directly without
acquiring a mem_access reference, triggering an assertion inside
xe_gt_idle_disable_c6.

Fixes: 975e4a37 ("drm/xe: Manually setup C6 when skip_guc_pc is set")
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240126220613.865939-2-matthew.d.roper@intel.com

drivers/gpu/drm/xe/xe_guc_pc.c

@@ -963,7 +963,9 @@ void xe_guc_pc_fini(struct xe_guc_pc *pc)
 	struct xe_device *xe = pc_to_xe(pc);
 
 	if (xe->info.skip_guc_pc) {
+		xe_device_mem_access_get(xe);
 		xe_gt_idle_disable_c6(pc_to_gt(pc));
+		xe_device_mem_access_put(xe);
 		return;
 	}