drm/i915/selftests: Flush all tiles on test exit

We want to idle all tiles when exiting selftests.

Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230125100003.18243-1-nirmoy.das@intel.com