drm/nouveau/therm/fan/tach: default to 2 pulses per revolution

I spent some time this weekend trying to find in the vbios the number of
pulses per revolutions in the vbios but couldn't find it. It would seem
all my cards have 2 pulses per revolution so let's stick to that until
further notice.

Thermal table's id 0x48 may indicate this information but it would seem
that changing the value results in the blob power or clock gating the
RPM counter... We should ask NVIDIA about that, should be trivial-enough
for them to answer.
Signed-off-by: Martin Peres <martin.peres@free.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/subdev/therm/nva3.c

@@ -36,7 +36,7 @@ nva3_therm_fan_sense(struct nouveau_therm *therm)
 	u32 tach = nv_rd32(therm, 0x00e728) & 0x0000ffff;
 	u32 ctrl = nv_rd32(therm, 0x00e720);
 	if (ctrl & 0x00000001)
-		return tach * 60;
+		return tach * 60 / 2;
 	return -ENODEV;
 }