backends/drm: limit max_bpc to 8 by default with docks
There's generally a lot of problems with higher than 8bpc in docking stations, especially when multiple monitors are involved. Until these problems are hopefully eventually fixed on the driver side, limit the bpc to 8 with docks by default
(cherry picked from commit 099a1cc3)
Co-authored-by: Xaver Hugl xaver.hugl@gmail.com