PCI/VGA: Use pci_is_vga() to identify VGA devices

Use pci_is_vga() to identify VGA devices, so the arbiter will handle old
PCI_CLASS_NOT_DEFINED_VGA (0x0001) devices as well as the
PCI_CLASS_DISPLAY_VGA (0x0300) devices it previously handled.

Link: https://lore.kernel.org/r/20230830111532.444535-3-sui.jingfeng@linux.dev


Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
[bhelgaas: commit log, split functional change from optimization]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: "Maciej W. Rozycki" <macro@orcam.me.uk>