drm/nouveau/pci/msi: disable MSI on big-endian platforms by default

It appears that MSI does not work on either G5 PPC nor on a E5500-based
platform, where other hardware is reported to work fine with MSI.

Both tests were conducted with NV4x hardware, so perhaps other (or even
this) hardware can be made to work. It's still possible to force-enable
with config=NvMSI=1 on load.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: stable@vger.kernel.org
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>