drm/xe/irq: Don't try to lookup engine masks for non-existent primary GT

drm/xe/irq: Don't try to lookup engine masks for non-existent primary GT

If the primary GT is disabled via configfs, we shouldn't try to access
it to lookup BCS/CCS engine masks.  For the purposes of IRQ reset (which
masks & disables interrupts in an sgunit register), assume all possible
instances are present.

Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Link: https://lore.kernel.org/r/20251013200944.2499947-39-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>