drm/panel: simple: Add missing .bpc fields

Various panels were missing the .bpc field which encodes the number of
bits per color. Not every display driver relies on this value, but since
the panels can be used with any display engine it must be specified so
that if a driver knows how to differentiate based on this field it can
do so.

Signed-off-by: Thierry Reding <treding@nvidia.com>