drm/xe: Bypass Wa_14018094691 when primary GT is disabled

Don't try to lookup Wa_14018094691 on a NULL GT when the primary GT is
disabled. Since this whole workaround centers around mid-thread
preemption behavior, the workaround isn't relevant if the primary GT
(where the engines that can do MTP live) is disabled.

Reviewed-by: Gustavo Sousa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Matt Roper <[e-mail]>

drivers/gpu/drm/xe/xe_gsc.c

···
 	unsigned int fw_ref;
 	int ret;
 
-	if (XE_GT_WA(tile->primary_gt, 14018094691)) {
+	if (tile->primary_gt && XE_GT_WA(tile->primary_gt, 14018094691)) {
 		fw_ref = xe_force_wake_get(gt_to_fw(tile->primary_gt), XE_FORCEWAKE_ALL);
 
 		/*
···
 
 	ret = gsc_upload(gsc);
 
-	if (XE_GT_WA(tile->primary_gt, 14018094691))
+	if (tile->primary_gt && XE_GT_WA(tile->primary_gt, 14018094691))
 		xe_force_wake_put(gt_to_fw(tile->primary_gt), fw_ref);
 
 	if (ret)