drm: rockchip: remove reference to non-existing config DRM_RGB

commit 1f0f0151 ("drm/rockchip: Add support for Rockchip Soc RGB
output interface") accidently adds to select the non-existing config
DRM_RGB in ./drivers/gpu/drm/rockchip/Kconfig.

Luckily, ./scripts/checkkconfigsymbols.py warns on non-existing configs:

DRM_RGB
Referencing files: drivers/gpu/drm/rockchip/Kconfig

So, remove the reference to the non-existing config DRM_RGB.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210819112253.16484-2-lukas.bulwahn@gmail.com