drm/nv50: Improve PGRAPH interrupt handling.

This makes nouveau recognise and report more kinds of PGRAPH errors, as
well as prevent GPU lockups resulting from some of them.

Lots of guesswork was involved and some part of this is probably
incorrect. Some potential-lockuop situations are handled by just
resetting a whole PGRAPH subunit, which doesn't sound like a "proper"
solution, but seems to work just fine... for now.

Signed-off-by: Marcin Kościelnicki <koriakin@0x04.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>