drm/i915: Add comments explaining the vdd on/off functions

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: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>