drm/gma500/mrst: Add aux register writes when programming pipe

On SDVO pipes (always Pipe B on mrst) we have to sequentially write the
aux vdc. We might be able to skip programming the primary vdc in
some/most places but we don't care about that now.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>