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

Use pci_is_vga() to identify VGA devices instead of a private is_vga()
function.

This means qxl will use the VGA arbiter for old PCI_CLASS_NOT_DEFINED_VGA
(0x0001) devices as well as the PCI_CLASS_DISPLAY_VGA (0x0300) devices it
recognized previously.

This probably doesn't make a difference because qxl_pci_driver doesn't
claim PCI_CLASS_NOT_DEFINED_VGA devices by default, so it's mainly a code
simplification.

Link: https://lore.kernel.org/r/20230830111532.444535-6-sui.jingfeng@linux.dev


Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>