vgaarb: Stub vga_set_legacy_decoding()

vga_set_legacy_decoding() is defined in drivers/gpu/vga/vgaarb.c,
which is only compiled with CONFIG_VGA_ARB. A caller would
therefore get an undefined symbol if the VGA arbiter is not
enabled.

Signed-off-by: Alex Williamson <[e-mail]>
Acked-by: Dave Airlie <[e-mail]>

include/linux/vgaarb.h

···
  *     out of the arbitration process (and can be safe to take
  *     interrupts at any time.
  */
+#if defined(CONFIG_VGA_ARB)
 extern void vga_set_legacy_decoding(struct pci_dev *pdev,
 				    unsigned int decodes);
+#else
+static inline void vga_set_legacy_decoding(struct pci_dev *pdev,
+					   unsigned int decodes) { };
+#endif
 
 /**
  *     vga_get         - acquire & locks VGA resources