drm/nouveau/disp/nv50-: prevent oops when no channel method map provided

[ Upstream commit 0e6176c6

 ]

The implementations for most channel types contains a map of methods to
priv registers in order to provide debugging info when a disp exception
has been raised.

This info is missing from the implementation of PIO channels as they're
rather simplistic already, however, if an exception is raised by one of
them, we'd end up triggering a NULL-pointer deref.  Not ideal...

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=206299
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>