drm/i915: Unconditionally clear the pm/guc GT IIR upon acking

Having stored the IIR for action, we should always clear it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180802100631.31305-2-chris@chris-wilson.co.uk