drm/nouveau/mxm: call mxmi to determine revision before calling mxms

There's a HP laptop out there where the MXM version in the VBIOS doesn't
match what the ACPI implementation is expecting.  These tables will accept
0x00 to MXMS to return latest version, but *only* if MXMI has been called
first..

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