drm/rockchip: vop: add the definition of dclk_pol

Some VOP's (such as px30) dclk_pol bit is at the last.
So it is necessary to distinguish dclk_pol and pin_pol.

Signed-off-by: Nickey Yang <nickey.yang@rock-chips.com>
Reviewed-by: Sandy Huang <hjc@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20191010034452.20260-2-nickey.yang@rock-chips.com