drm/i915/wm: move watermark debugfs to intel_wm.c

Follow the new convention of placing debugfs with the code.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215141910.433043-3-jani.nikula@intel.com