There are two leftover GTIIR writes in valleyview_irq_preinstall().
Looks like the were originally left behind by:

 commit d18ea1b5
 Author: Daniel Vetter <daniel.vetter@ffwll.ch>
 Date:   Fri Jul 12 22:43:25 2013 +0200

    drm/i915: unify PM interrupt preinstall sequence

and then the GTIIR reset was added back here:

 commit f86f3fb0
 Author: Paulo Zanoni <paulo.r.zanoni@intel.com>
 Date:   Tue Apr 1 15:37:14 2014 -0300

    drm/i915: properly clear IIR at irq_uninstall on Gen5+

so we can kill the leftovers from the vlv code.

Cc: Paulo Zanoni <przanoni@gmail.com>
Suggested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>