drm/i915: Call gen5_gt_irq_reset() from valleyview_irq_uninstall()

Looks like we forgot to call gen5_gt_irq_reset() for vlv in the
uninstall phase. Do so.

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