drm/virtio: Use pci_is_vga() to identify VGA devices

Use pci_is_vga() to identify VGA devices instead of open-coding the class
test.

This means virtio_gpu_pci_quirk() will apply to old
PCI_CLASS_NOT_DEFINED_VGA (0x0001) devices as well as the
PCI_CLASS_DISPLAY_VGA (0x0300) devices it did previously.

Link: https://lore.kernel.org/r/20230830111532.444535-5-sui.jingfeng@linux.dev


Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: David Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Gurchetan Singh <gurchetansingh@chromium.org>
Cc: Chia-I Wu <olvaffe@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>