drm/pl111: Handle the RealView variant separately

We want to cut down the default bpp to 16 on the RealView so
we can have a 1024x768 framebuffer console by default. The
memory bandwidth limitations makes this not work with the
PL111 default of 32bpp.

This builds on top of the earlier patches making the
framebuffer default bpp a per-variant variable.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180302090948.6399-4-linus.walleij@linaro.org