drm/xe: Assign GT IDs properly on multi-tile + multi-GT platforms

Although "multi-tile" and "multiple GTs per tile" are mutually-exclusive
characteristics on all of our platforms today, this may not always be
true.  Assign GT IDs according to xe->info.max_gt_per_tile in a way that
should work even if future platforms have different configurations.

This patch should not change the behavior of current platforms; it only
future-proofs for potential future designs.

v2:
 - Re-calculate gt_count if tile count gets reduced by MTCFG.  (PVC CI)

Reviewed-by: Ravi Kumar Vodapalli <ravi.kumar.vodapalli@intel.com>
Link: https://lore.kernel.org/r/20250701201320.2514369-14-matthew.d.roper@intel.com


Signed-off-by: Matt Roper <matthew.d.roper@intel.com>