drm/nouveau/kms: Increase max retries in scanout position queries.

So far we only allowed for 1 retry and just failed the query
- and thereby high precision vblank timestamping - if we did
not get a reasonable result, as such a failure wasn't considered
all too horrible. There are a few NVidia gpu models out there which
may need a bit more than 1 retry to get a successful query result
under some conditions.

Since Linux 4.4 the update code for vblank counter and timestamp
in drm_update_vblank_count() changed so that the implementation
assumes that high precision vblank timestamping of a kms driver
either consistently succeeds or consistently fails for a given
video mode and encoder/connector combo. Iow. switching from success
to fail or vice versa on a modeset or connector change is ok, but
spurious temporary failure for a given setup can confuse the core
code and potentially cause bad miscounting of vblanks and confusion
or hangs in userspace clients which rely on vblank  stuff, e.g.,
desktop compositors.

Therefore change the max retry count to a larger number - more than
any gpu so far is known to need to succeed, but still low enough
so that these queries which do also happen in vblank interrupt are
still fast enough to be not disastrously long if something would
go badly wrong with them.

As such sporadic retries only happen seldom even on affected gpu's,
this could mean a vblank irq could take a few dozen microseconds
longer every few hours of uptime -- better than a desktop compositor
randomly hanging every couple of hours or days of uptime in a hard
to reproduce manner.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_display.c

@@ -111,7 +111,7 @@ nouveau_display_scanoutpos_head(struct drm_crtc *crtc, int *vpos, int *hpos,
 	};
 	struct nouveau_display *disp = nouveau_display(crtc->dev);
 	struct drm_vblank_crtc *vblank = &crtc->dev->vblank[drm_crtc_index(crtc)];
-	int ret, retry = 1;
+	int ret, retry = 20;
 
 	do {
 		ret = nvif_mthd(&disp->disp, 0, &args, sizeof(args));