hw/display/exynos4210_fimd: Fix potential NULL pointer dereference

In exynos4210_fimd_update(), the pointer s is dereferinced before
being check if it is valid, which may lead to NULL pointer dereference.
So move the assignment to global_width after checking that the s is valid.

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Alex Chen <alex.chen@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 5F9F8D88.9030102@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>