drm/nouveau: fix regression in vblanking

nv50_vblank_enable/disable got switched from NV50_PDISPLAY_INTR_EN_1_VBLANK_CRTC_0 (4) << head to 1 << head, which is wrong.

4 << head is the correct value.

Fixes regression with vblanking since 1d7c71a3 "drm/nouveau/disp: port vblank handling to event interface"
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/core/engine/disp/nv50.c

@@ -544,13 +544,13 @@ nv50_disp_curs_ofuncs = {
 static void
 nv50_disp_base_vblank_enable(struct nouveau_event *event, int head)
 {
-	nv_mask(event->priv, 0x61002c, (1 << head), (1 << head));
+	nv_mask(event->priv, 0x61002c, (4 << head), (4 << head));
 }
 
 static void
 nv50_disp_base_vblank_disable(struct nouveau_event *event, int head)
 {
-	nv_mask(event->priv, 0x61002c, (1 << head), (0 << head));
+	nv_mask(event->priv, 0x61002c, (4 << head), 0);
 }
 
 static int