drm/i915/debugfs: Enable upper layer interfaces to act on all gt's

The commit 82a149a6

 ("drm/i915/gt: move remaining debugfs
interfaces into gt") moved gt-related debugfs files in the gtX/
directories to operate on individual gt's.

However, the original files were only functioning on the root
GT (GT 0) and have been left in the same location to maintain
compatibility with userspace users.

Add multiplexing functionality to the higher directories' files.
This enables the operations to be performed on all the GTs with
a single write. In the case of reads, the files provide an or'ed
value across all the tiles.

Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Maciej Patelczyk <maciej.patelczyk@intel.com>
Reviewed-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230318203616.183765-4-andi.shyti@linux.intel.com