drm/i915/selftests: Small tweak to put the termination conditions together

If we run out of ring space, or exceed the desired runtime, we wish to
stop the subtest. Put these checks together, so that we always keep the
requests flushed on completion.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120140314.24749-3-chris@chris-wilson.co.uk