drm/xe: Use for_each_gt to define gt_count

We are currently bumping gt_count as we define GTs for each tile. While
that works with our current code, there are reasons to improve that:

* That section of the code is dedicated to define each tile's set of GTs
  and their respective info. The gt_count can be seen as a device level
  property. While it is fair to bump it as we define each GT, we can
  also focus on the GT themselves and count after we are done with the
  definitions.

* More *importantly*, gt_count should reflect the number of GTs that we
  would get when looping over them with for_each_gt(). Bumping gt_count
  the way we are currently doing makes that value not really connected
  to for_each_gt(). This could bite us in the future if in the loop gets
  extra checks that are not accounted for in each existing "gt_count++".

As such, let's use for_each_gt() and extract the calculation of gt_count
into a separate block, just after we define the set of GTs for each
tile.

Reviewed-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://lore.kernel.org/r/20250818-gt_count-improvements-v4-2-ee12870c6f57@intel.com


Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>