drm/i915: Add comments about units of latency values

All the ILK+ WM compute functions take the latency values in 0.1us
units. Add a few comments to remind people about that.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>