video: Add 32-bit color depth support for VBE

The TunnelCreek IGD VBE reports 32-bit color depth regardless 24-bit
color depth is configured. Since 24-bit mode already uses 4 bytes
internally, it should be OK to just add this option in switch case.

Signed-off-by: Jian Luo <jian.luo4@boschrexroth.de>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Bin Meng <bmeng.cn@gmail.com>