drm/rockchip: vop: Fix a dereferenced before check warning

The 'state' can't be NULL, we should check crtc_state.

Fix warning:
drivers/gpu/drm/rockchip/rockchip_drm_vop.c:1096
vop_plane_atomic_async_check() warn: variable dereferenced before check
'state' (see line 1077)

Fixes: 5ddb0bd4 ("drm/atomic: Pass the full state to planes async atomic check and update")
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20241021072818.61621-1-andyshrk@163.com