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

BugLink: https://bugs.launchpad.net/bugs/1868627

[ 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=206299Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kelsey Skunberg <kelsey.skunberg@canonical.com>

drivers/gpu/drm/nouveau/nvkm/engine/disp/channv50.c

@@ -72,6 +72,8 @@ nv50_disp_chan_mthd(struct nv50_disp_chan *chan, int debug)
 
 	if (debug > subdev->debug)
 		return;
+	if (!mthd)
+		return;
 
 	for (i = 0; (list = mthd->data[i].mthd) != NULL; i++) {
 		u32 base = chan->head * mthd->addr;