drm/nouveau/mxm: pretend to succeed, even if we can't shadow the MXM-SIS

There's at least one known case where our shadowing code is buggy, and we
fail init.  Until we can be confident we're doing all this correctly, lets
succeed and risk crazy bios tables rather than failing for perfectly valid
configs too.

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