drm/i915: Add a wrapper for frequency debugfs

Move it to the RPS source file.

v2: Separate out code movement and functional changes (Jani)

Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221005155943.34747-2-vinay.belgaumkar@intel.com