PCI: Add pci_is_vga() helper

The PCI Code and ID Assignment spec, r1.15, secs 1.4 and 1.1, define
VGA Base Class and Sub-Classes:

  03 00  PCI_CLASS_DISPLAY_VGA      VGA-compatible or 8514-compatible
  00 01  PCI_CLASS_NOT_DEFINED_VGA  VGA-compatible (before Class Code)

Add a pci_is_vga() helper to return true if a device is in either category.

These VGA devices use the hardwired legacy VGA resources
([mem 0xa0000-0xbffff], [io 0x3b0-0x3bb], [io 0x3c0-0x3df] and aliases), so
they require special handling if more than one is present in the system.

Link: https://lore.kernel.org/r/20230830111532.444535-2-sui.jingfeng@linux.dev
Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
[bhelgaas: commit log, drop !pdev test]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: "Maciej W. Rozycki" <macro@orcam.me.uk>

Author: Sui Jingfeng

include/linux/pci.h

@@ -713,6 +713,30 @@ static inline bool pci_is_bridge(struct pci_dev *dev)
 		dev->hdr_type == PCI_HEADER_TYPE_CARDBUS;
 }
 
+/**
+ * pci_is_vga - check if the PCI device is a VGA device
+ *
+ * The PCI Code and ID Assignment spec, r1.15, secs 1.4 and 1.1, define
+ * VGA Base Class and Sub-Classes:
+ *
+ *   03 00  PCI_CLASS_DISPLAY_VGA      VGA-compatible or 8514-compatible
+ *   00 01  PCI_CLASS_NOT_DEFINED_VGA  VGA-compatible (before Class Code)
+ *
+ * Return true if the PCI device is a VGA device and uses the legacy VGA
+ * resources ([mem 0xa0000-0xbffff], [io 0x3b0-0x3bb], [io 0x3c0-0x3df] and
+ * aliases).
+ */
+static inline bool pci_is_vga(struct pci_dev *pdev)
+{
+	if ((pdev->class >> 8) == PCI_CLASS_DISPLAY_VGA)
+		return true;
+
+	if ((pdev->class >> 8) == PCI_CLASS_NOT_DEFINED_VGA)
+		return true;
+
+	return false;
+}
+
 #define for_each_pci_bridge(dev, bus)				\
 	list_for_each_entry(dev, &bus->devices, bus_list)	\
 		if (!pci_is_bridge(dev)) {} else