drm/xe: Bypass Wa_14018094691 when primary GT is disabled

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 <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/20251013200944.2499947-42-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>