PCI/sysfs: Enable 'boot_vga' attribute via pci_is_vga()

Enable the 'boot_vga' sysfs attribute via pci_is_vga().

This exposes 'boot_vga' for old PCI_CLASS_NOT_DEFINED_VGA (0x0001) devices
as well as for the PCI_CLASS_DISPLAY_VGA (0x0300) devices where it was
previously exposed.

Link: https://lore.kernel.org/r/20230830111532.444535-4-sui.jingfeng@linux.dev


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