drm/i915: Move DRRS debugfs next to the implementation

Move the DRRS debugfs stuff next to the actual implementation
so that it's easier to deal with the whole.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221003113249.16213-2-ville.syrjala@linux.intel.com