drm/xe: Break GT setup out of xe_info_init()

drm/xe: Break GT setup out of xe_info_init()

xe_info_init() is getting a bit long and hard to follow.  Break the
allocation and basic initialization of the xe_gt structures out to their
own functions.

v2:
 - Rename new functions from init_* to alloc_*.  (Gustavo)
 - Move early NULL return of media GT before allocation.  (Gustavo)

Cc: Gustavo Sousa <gustavo.sousa@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Gustavo Sousa <gustavo.sousa@intel.com>
Link: https://lore.kernel.org/r/20251013200944.2499947-46-matthew.d.roper@intel.com
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>