drm/xe: Check that GT is not NULL before testing Wa_16023588340

If the primary GT is disabled, skip the check for this workaround (which
only applies to dgpu platforms where the primary GT cannot be NULL).

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

drivers/gpu/drm/xe/display/xe_display_wa.c

···
 bool intel_display_needs_wa_16023588340(struct intel_display *display)
 {
 	struct xe_device *xe = to_xe_device(display->drm);
+	struct xe_gt *wa_gt = xe_root_mmio_gt(xe);
 
-	return XE_GT_WA(xe_root_mmio_gt(xe), 16023588340);
+	return wa_gt && XE_GT_WA(wa_gt, 16023588340);
 }