drm/nouveau/mc/ga10[024]: initial support

Fortunately, all the interrupts we need to bring up basic display support
are contained in a single leaf register, allowing this basic (but hackish)
implementation.

There's a bunch more invasive patches to come implementing all this in a
better/more complete way, but trying to get a minimal series out first.

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