drm/i915: Set DP min_bpp to 8*3 for non-RGB output formats

6bpc is only legal for RGB and RAW pixel encodings. For the rest
the minimum is 8bpc. Set our lower limit accordingly.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326142556.21176-6-ville.syrjala@linux.intel.com


Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>