drm/gma500/mrst: Add aux register writes to SDVO

This turned out to be tricky. Writing to SDVOB on the primary vdc also
writes to SDVOB on the aux vdc, but reading it back on the primary vdc
always fails. Basically we never read from the primary vdc since we
will end up trashing the aux vdc.

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