drm/i915: Clear all residual RPS events on disabling interrupts

Make sure that the RPS IIR is completely clear on disabling so we should
not get any more interrupts after idling. Since the IIR is shared with
the guc, we have to be careful to only clobber RPS events.

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/20180802100631.31305-3-chris@chris-wilson.co.uk