If crtc <-> encoder linkage changes, we could end up with the CRTC
listening for the wrong error or vsync irqs.  Generally this problem
would correct itself relatively quickly, since we update the global
irqmask after dispatching irqs, but to be sure let the CRTC trigger
update_irq().
Signed-off-by: NRob Clark <robdclark@gmail.com>