drm/i915/gt: create per-tile sysfs interface

Now that we have tiles we want each of them to have its own
interface. A directory "gt/" is created under "cardN/" that will
contain as many diroctories as the tiles.

In the coming patches tile related interfaces will be added. For
now the sysfs gt structure simply has an id interface related
to the current tile count.

The directory structure will follow this scheme:

    /sys/.../card0
             └── gt
                 ├── gt0
                 │   └── id
                 :
		 :
		 └─- gtN
                     └── id

This new set of interfaces will be a basic tool for system
managers and administrators when using i915.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Sujaritha Sundaresan <sujaritha.sundaresan@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Sujaritha Sundaresan <sujaritha.sundaresan@intel.com>
Reviewed-by: Andrzej Hajda <andrzej.hajda@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220318233938.149744-5-andi.shyti@linux.intel.com