drm/nvd0/disp: move syncs/magic setup to or mode_set

NVIDIA appear to do these around the same place they do the MODE_CTRL
methods, and for DP at least we need to bash some extra bits in "syncs"
to keep EVO happy.

It's a bit of a guess as to the 6/8bpc, but i have no better idea yet.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>