drm/nouveau: fix pcirom vbios shadow breakage from acpi rom patch

On nv50 it became impossible to attempt a PCI ROM shadow of the VBIOS,
which will break some setups.

This patch also removes the different ordering of shadow methods for
pre-nv50 chipsets.  The reason for the different ordering was paranoia,
but it should hopefully be OK to try shadowing PRAMIN first.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>