drm/apple: Use preferred display mode from dcp

[nohenry]

The DCP display properties contains a preferred mode flag. This patch takes that into account when picking which mode to assign DRM_MODE_TYPE_PREFERRED

[jannau]

Please mention explicitly in the commit message that the mode with the best score isn't necessary a preferred mode and that using one of the preferred modes is a better match for some displays. Example with a few modes from your display would be nice.

Implementation wise I wonder whether it would make sense to just add 1 << 32 to the score for preferred mode. That would avoid making the preferred mode selection more complex.