drm/panel: simple: Add bits-per-color support

The bits-per-color is provided by the EDID normally, but if we're using
panels, we need to store it somewhere. So we add a field to the panel
descriptor for it.

Signed-off-by: Stéphane Marchesin <marcheu@chromium.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>