drm/pl111: Support grayscale

Migrating the TI nspire calculators to use the PL111 driver for
framebuffer requires grayscale support for the elder panel
which uses 8bit grayscale only.

DRM does not support 8bit grayscale framebuffers in memory,
but by defining the bus format to be MEDIA_BUS_FMT_Y8_1X8 we
can get the hardware to turn on a grayscaling feature and
convert the RGB framebuffer to grayscale for us.

Cc: Daniel Tang <dt.tangr@gmail.com>
Cc: Fabian Vogt <fabian@ritter-vogt.de>
Tested-by: Fabian Vogt <fabian@ritter-vogt.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190805085847.25554-2-linus.walleij@linaro.org