drm/nvd0/disp: hopefully fix selection of 6/8bpc mode on DP outputs

I have a very limited number of traces available for DP on NVD9+, but,
these values produce the same as the binary driver on a confirmed 18-bit
eDP panel and a confirmed 24-bit eDP panel (Retina MBP).

It's interesting that the bitfield values also match the MODE_CTRL values
that control the same thing on nv50:nvd9.

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