drm/i915/vlv: move the vlv PLL helper next to its platform counterparts

Move the helper next to the PLL helpers of the other platforms for
clarity.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Sonika Jindal <sonika.jindal@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>