drm/nouveau: protect ramht_find() from oopsing if on channel without ramht

This doesn't actually happen now, but there's a test case for an earlier
kernel where a GPU error is signalled on one of nv50's fake channels, and
the ramht lookup by the IRQ handler triggered an oops.

This adds a check for RAMHT's existance on a channel before looking up
an object handle.

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