drm/rockchip: Make VOP driver optional

With upcoming VOP2 support VOP won't be the only choice anymore, so make
the VOP driver optional.

This also adds a dependency from ROCKCHIP_ANALOGIX_DP to ROCKCHIP_VOP,
because that driver currently only links and works with the VOP driver.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220422072841.2206452-22-s.hauer@pengutronix.de