drm/i915: split clock gating init from display vtable

This function is only used inside intel_pm.c

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/95d626a7329ab5779804762894e304e12c6dbe1f.1632869550.git.jani.nikula@intel.com