Jani wanted some comments to explain why we call certain vdd on/off
functions in certain places.

v2: Make the comments more thorough (Imre)
Reviewed-by: NImre Deak <imre.deak@intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>