drm/xe: Fix GT looping for standalone media

drm/xe: Fix GT looping for standalone media

gt_count is only being incremented when initializing the primary GT;
since the media GT sets the ID directly, gt_count is not incremented
again, resulting in an incorrect count on MTL.  Use autoincrement while
assigning the media GTs ID to ensure gt_count is correct on MTL and
other future platforms with standalone media.

Signed-off-by: Riana Tauro <riana.tauro@intel.com>
Link: https://lore.kernel.org/r/20230613094232.3703549-1-riana.tauro@intel.com
[mattrope: Tweaked commit message to focus on gt_count importance]
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>