drm/pl111: Make the default BPP a per-variant variable

The PL110, Integrator and Versatile boards strongly prefer to
use 16 BPP even if other modes are supported, both to keep down
memory consumption and also to easier find a good match to
supported resolutions with consideration taken to the memory
bandwidth of the platforms.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180302090948.6399-2-linus.walleij@linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20180307084316.23623-1-linus.walleij@linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20180307084316.23623-1-linus.walleij@linaro.org