drm/nv50: Keep track of the head a channel is vsync'ing to.

In a multihead setup vblank interrupts may end up enabled in both
heads. In that case we want to ignore the vblank interrupts coming
from the wrong CRTC to avoid tearing and unbalanced calls to
drm_vblank_get/put (fdo bug 31074).

Reported-by: Felix Leimbach <felix.leimbach@gmx.net>
Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>