drm/xe: Add clearing stats to GT debugfs

It helpful to clear GT stats, run a test cases which is being profiled,
and look at the results of the stats from the individual test case. Make
stats entry writable and upon write clear the stats.

v5:
 - Drop clear_stats debugfs entry (Lucas)
v6:
 - Use xe_gt_stats_clear rather than helper (Michal)
 - Rework loop in xe_gt_stats_clear (Michal)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://lore.kernel.org/r/20250829172232.1308004-2-matthew.brost@intel.com