The VGA arbiter is really PCI-specific and doesn't depend on any GPU
things.  Move it to the PCI subsystem.

Note that misc_init() must be called before vga_arb_device_init().  These
are both subsys_initcalls, so this ordering depends on the link order,
which is determined by drivers/Makefile:

  obj-y += pci/
  obj-y += char/        <-- misc_init()
  obj-y += gpu/         <-- vga_arb_device_init() (before this commit)

The drivers/pci/ subsys_initcalls are called *before* misc_init(), so
convert vga_arb_device_init() to subsys_initcall_sync(), which is called
after *all* subsys_initcalls.

Link: https://lore.kernel.org/r/20220224224753.297579-2-helgaas@kernel.orgSigned-off-by: Bjorn Helgaas <bhelgaas@google.com>