drm/xe/pmu: Initialize PMU event types based on first available GT

drm/xe/pmu: Initialize PMU event types based on first available GT

GT ID#0 (primary GT on tile 0) may not always be available if the
primary GT has been disabled via configfs.  Instead use the first
available GT when determining which PMU events are supported.  If there
are no GTs, then don't advertise any GT-related events.

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20251013200944.2499947-36-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>