drm/nouveau/intr: support multiple trees, and explicit interfaces

Turing adds a second top-level interrupt tree in HW, in addition to the
trees available via NV_PMC.  Most of the interrupts we care about are
exposed in both trees, but not all of them, and we have some rather
nasty hacks to route the fault buffer interrupts.

Ampere removes the NV_PMC trees entirely.

Here we add some infrastructure to be able to handle all of this more
cleanly, as well as providing more explicit control over handlers.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>