Some drivers (e.g. sun4i-drm) need this info to decide whether they
need to enable dithering. Currently driver reports what panel supports
and if panel supports 8 we don't get dithering enabled.

Hardcode BPC to 6 for now since that's the only BPC
that driver supports.

Fixes: 6aa19269 ("drm/bridge: Add Analogix anx6345 support")
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Acked-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200329222253.2941405-1-anarsoul@gmail.com