drm/i915: rename debugfs_gt files

We shouldn't be using debugfs_ namespace for this functionality. Rename
debugfs_gt.[ch] to intel_gt_debugfs.[ch] and then make functions,
defines and structs follow suit.

While at it and since we are renaming the header, sort the includes
alphabetically.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210918025754.1254705-1-lucas.demarchi@intel.com