drm/i915: synchronize_irq() before turning off disp2d power well on VLV/CHV

drm/i915: synchronize_irq() before turning off disp2d power well on VLV/CHV

After we've told the irq code we don't want to handle display irqs
anymore, we must make sure any display irq handling already
kicked off has finished before we actually turn off the power well.

I wouldn't expect PIPESTAT based interrupts to occur anymore since
vblanks/page flips/gmbus/etc should all be quiescent at this point.
But at least hotplug interrupts could still occur. Hotplug
interrupts may also kick off the workqueue based hotplug processing,
but that code should take the required power domain references
itself, so there shouldn't be any need to synchronize with the
hotplug processing from the power well code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455900112-15387-1-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>