drm/udl: Set preferred color depth to 16 bpp

The current default color depth of 24 bpp is not even supported by
the driver. Being the native format for communicating with the adapter,
16 bpp is the correct choice.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191210084905.5570-6-tzimmermann@suse.de