PCI: Mark shadow copy of VGA ROM as IORESOURCE_PCI_FIXED

A shadow copy of an option ROM is placed by the BIOS as a fixed address.
Set IORESOURCE_PCI_FIXED to indicate that we can't move the shadow copy.
This prevents warnings like the following when we assign resources:

  BAR 6: [??? 0x00000000 flags 0x2] has bogus alignment

This warning is emitted by pdev_sort_resources(), which already ignores
IORESOURCE_PCI_FIXED resources.

Link: http://lkml.kernel.org/r/CA+55aFyVMfTBB0oz_yx8+eQOEJnzGtCsYSj9QuhEpdZ9BHdq5A@mail.gmail.com
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>