x86: qemu: Adjust VGA initialization

As VGA option rom needs to run at C segment, although QEMU PAM emulation
seems to only guard E/F segments, for correctness, move VGA initialization
after PAM decode C/D/E/F segments.

Also since we already tested QEMU targets to differentiate I440FX and Q35
platforms, change to locate the VGA device via hardcoded b.d.f instead of
dynamic search for its vendor id & device id pair.

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