drm/rockchip: vop: Add directly output rgb feature for px30

Add this feature bit indicate px30 vop can directly output
parallel or serial rgb data.

Signed-off-by: Sandy Huang <hjc@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180830211207.10480-4-heiko@sntech.de