nouveau: check valid DP modes always with 6 bpc
This allows nouveau to advertise 4K@60 modes on my 4K HDR display connected through an HDMI to DP converter on DP 1.2 capable GPUs.
Connecting it with DP direclty works without this modification, the reason simply is that with HDMI we get 597 MHz modes, with DP 533 MHz ones.