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>