drm/i915: Eliminate loop from VLV irq handler

Now that we've dealt with the races in clearing IIR bits via VLV_IER
and the master interrupt enable, we can go ahead aliminate the loop
from the VLV interrupt handler.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460571598-24452-7-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>