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>

Author: Sui Jingfeng

drivers/gpu/drm/virtio/virtgpu_drv.c

@@ -51,7 +51,7 @@ static int virtio_gpu_pci_quirk(struct drm_device *dev)
 {
 	struct pci_dev *pdev = to_pci_dev(dev->dev);
 	const char *pname = dev_name(&pdev->dev);
-	bool vga = (pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA;
+	bool vga = pci_is_vga(pdev);
 	int ret;
 
 	DRM_INFO("pci: %s detected at %s\n",