video: fbdev: via: remove VLA usage

In preparation to enabling -Wvla, remove VLA usage.

Also, fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621



Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>