powerpc: Set default VGA device

Add a PCI quirk for VGA devices on Power to set the default VGA device.
Ensures a default VGA is always set if a graphics adapter is present,
even if firmware did not initialize it. If more than one graphics
adapter is present, ensure the one initialized by firmware is set
as the default VGA device. This ensures that X autoconfiguration
will work.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>