drm/i915/icl: Don't warn on spurious interrupts

There is a chance we can see spurious interrupts in live
now. We have more engines enabled and that with more elaborate
access patterns with pm and display, increases the chances
hardware just makes a social call, without anything to work on.

Remove the error as we have tests to actually probe if
we really miss interrupt, instead of getting spurious ones.

Note that now we do write to intr_dw even with a zero
value. This is considered advantegous as the write
is an ack that sw is done.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190410132124.21795-2-mika.kuoppala@linux.intel.com