drm: fsl-dcu: Fix no fb check bug

For state->fb or state->crtc may be NULL in fsl_dcu_drm_plane_atomic_check
function, if so, return 0.

Signed-off-by: Meng Yi <meng.yi@nxp.com>
Signed-off-by: Jianwei Wang <jianwei.wang.chn@gmail.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>