drm/i915/gt: Create per-tile RC6 sysfs interface

Now tiles have their own sysfs interfaces under the gt/
directory. Because RC6 is a property that can be configured on a
tile basis, then each tile should have its own interface

The new sysfs structure will have a similar layout for the 4 tile
case:

/sys/.../card0
├── gt
│   ├── gt0
│   │   ├── id
│   │   ├── rc6_enable
│   │   ├── rc6_residency_ms
. . .
. . .
. .
│   └── gtN
│   ├── id
│   ├── rc6_enable
│   ├── rc6_residency_ms
│ .
│ .
│
└── power/ -+
├── rc6_enable | Original interface
├── rc6_residency_ms +-> kept as existing ABI;
. | it multiplexes over
. | the GTs
-+

The existing interfaces have been kept in their original location
to preserve the existing ABI. They act on all the GTs: when
reading they provide the average value from all the GTs.

This patch is not really adding exposing new interfaces (new
ABI) other than adapting the existing one to more tiles. In any
case 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>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Sujaritha Sundaresan <sujaritha.sundaresan@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@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/20220318233938.149744-6-andi.shyti@linux.intel.com