drm/i915/selftests: Flush partial-tiling object once

We only need to flush the object once prior to starting the partial
tiling test as inside the test we explicitly maintain coherency.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190604120022.20472-1-chris@chris-wilson.co.uk