drm/nouveau/nv46: Change mc subdev oclass from nv44 to nv4c

MSI interrupts appear to not work for nv46 based cards. Change the mc
subdev oclass for these cards from nv44 to nv4c, the nv4c mc code is
identical to the nv44 mc code except that it does not use msi
(it does not define a msi_rearm callback).

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=90435


Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>