video: vesa_fb: Look up VGA device by class instead of id

Per PCI spec, VGA device reports its class as standard 030000h in
its configuration space, so we can use it to determine if we need
run option rom instead of testing the supported vendor/device ids.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>