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Allow xe_device_get_gt() and for_each_gt() to operate as expected on
platforms with standalone media.

FIXME: We need to figure out a consistent ID scheme for GTs.  This patch
keeps the pre-existing behavior of 0/1 being the GT IDs for both PVC
(multi-tile) and MTL (multi-GT), but depending on the direction we
decide to go with uapi, we may change this in the future (e.g., to
return 0/1 on PVC and 0/2 on MTL).  Or if we decide we only need to
expose tiles to userspace and not GTs, we may not even need ID numbers
for the GTs anymore.

v2:
 - Restructure a bit to make the assertions more clear.
 - Clarify in commit message that the goal here is to preserve existing
   behavior; UAPI-visible changes may be introduced in the future once
   we settle on what we really want.
v3:
 - Store total GT count in xe_device for ease of lookup.  (Brian)
 - s/(id__++)/(id__)++/  (Gustavo)

Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Cc: Brian Welty <brian.welty@intel.com>
Acked-by: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20230601215244.678611-29-matthew.d.roper@intel.com

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