x86/PCI: Set IORESOURCE_ROM_SHADOW only for the default VGA device

Setting the IORESOURCE_ROM_SHADOW flag on a VGA card other than the primary
prevents it from reading its own ROM.  It will get the content of the
shadow ROM at C000 instead, which is of the primary VGA card and the driver
of the secondary card will bail out.

Fix this by checking if the arch code or vga-arbitration has already
determined the vga_default_device, if so only apply the fix to this primary
video device and let the comment reflect this.

[bhelgaas: add subject, split x86 & ia64 into separate patches]
Signed-off-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>