video: fbdev: Fix potential NULL pointer dereference

There is a potential NULL pointer dereference in case
fb_create_modedb() fails and returns NULL.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>