drm/rockchip: vop: Fix a dereferenced before check warning
In the Linux kernel, the following vulnerability has been resolved:

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)