drm/pl111: Properly detect the ARM PL110 variants

drm/pl111: Properly detect the ARM PL110 variants

With a bit of refactoring we can contain the variant data for
the strange PL110 versions that is feature-incomplete PL110 for
the ARM Integrator/CP and somewhere inbetween PL110 and PL111
for the ARM Versatile AB and Versatile PB.

We also accomodate for the custom duct-taped RGB565/BGR565 support
in the Versatile variant.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180206093540.8147-1-linus.walleij@linaro.org