drm/ingenic: Add support for 24-bit modes

Starting from the JZ4725B SoC, the primary and overlay planes support
24-bit pixel modes (8 bits per color component, without dummy byte).
Add support for these in the ingenic-drm driver.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200926170501.1109197-8-paul@crapouillou.net