drm/i915/gt: Create per-gt debugfs files

To support multi-GT configurations, we need to generate
independent debug files for each GT.

To achieve this create a separate directory for each GT under the
debugfs directory. For instance, in a system with two GTs, the
debugfs structure would look like this:

/sys/kernel/debug/dri
                  └── 0
                      ├── gt0
                      │   ├── drpc
                      │   ├── engines
                      │   ├── forcewake
                      │   ├── frequency
                      │   └── rps_boost
                      └── gt1
                      :   ├── drpc
                      :   ├── engines
                      :   ├── forcewake
                          ├── frequency
                          └── rps_boost

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230318203616.183765-2-andi.shyti@linux.intel.com