drm/i915: Shuffle some PLL code around

Shuffle some PLL functions around a bit to avoid ugle
forward declarations later on. No functional changes.

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/20220907091057.11572-3-ville.syrjala@linux.intel.com