gma500: Fix n, m1 and m2 clock limits for sdvo and lvds

The values of n, m1 and m2 needs to be subtracted by 2 before writing them to
the FP register. The dot clock calculation already thinks of these values in
register form so we must also specify them as such.

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