drm/i915/selftests: Delay spinner before waiting for an interrupt

It seems that although (perhaps because of the memory stall?) the
spinner has signaled that it has started, it still takes some time to
spin up to 100% utilisation of the HW. Since the test depends on the
full utilisation of the HW to trigger the RPS interrupt, wait a little
bit and flush the interrupt status to be sure that the event we see if
from the spinner.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200417093928.17822-1-chris@chris-wilson.co.uk