drm/i915: rename debugfs_gt_pm files

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

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/20210918025754.1254705-3-lucas.demarchi@intel.com