drm/i915/pm: hide struct drm_i915_clock_gating_funcs

The struct is only needed in intel_pm.c, move it there.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/63779ac0472895803113180818ecadc22c10369e.1643896905.git.jani.nikula@intel.com