Manually enable a 64GB 64-bit BAR so we have enough room for graphics
devices with large framebuffers.

Most BIOSes don't enable this for compatibility reasons.
Signed-off-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NAndy Shevchenko <andy.shevchenko@gmail.com>