drm/pl111: Handle the Versatile RGB/BGR565 mode

The ARM Versatile series can do RGB/BGR565 with an external
"PLD" (Programmable Logical Device). However the CLCD does not
have control bits for this, so it needs to be set into the
ordinary 16BPP mode, then the RGB/BGR565 handling of the pixel
data is handled by configuring the PLD through the external
register.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180206093540.8147-2-linus.walleij@linaro.org