drm/rockchip: vop2: fail cleanly if missing a primary plane for a video-port

Each window of a vop2 is usable by a specific set of video ports, so while
binding the vop2, we look through the list of available windows trying to
find one designated as primary-plane and usable by that specific port.

The code later wants to use drm_crtc_init_with_planes with that found
primary plane, but nothing has checked so far if a primary plane was
actually found.

For whatever reason, the rk3576 vp2 does not have a usable primary window
(if vp0 is also in use) which brought the issue to light and ended in a
null-pointer dereference further down.

As we expect a primary-plane to exist for a video-port, add a check at
the end of the window-iteration and fail probing if none was found.

Fixes: 604be855 ("drm/rockchip: Add VOP2 driver")
Reviewed-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250610212748.1062375-1-heiko@sntech.de