drm/ingenic: Add support for paletted 8bpp

On JZ4725B and newer, the F0 plane supports paletted 8bpp with a
256-entry palette. Add support for it.

v3: Only accept a full 256-entry palette.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200927193645.262612-2-paul@crapouillou.net