drm/i915: clarify gen2 pageflip cmd

I've reviewed gen2 pageflip code to hunt down multiple prepare pageflip
issues. The only thing I've found is a slight but functionally
meaningless confusion about the length of the mi cmd.

Fix it up and add a comment about what this dword should be (according
to docs at least).

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>