drm/i915/selftests: Replace wmb() with i915_gem_chipset_flush()

drm/i915/selftests: Replace wmb() with i915_gem_chipset_flush()

Currently, we are being fairly lazy and only using a wmb() following an
update to an active batch. Previously, we have found that to be
insufficient to ensure that a write from the CPU reaches memory in a
timely fashion, and in some caches we may need to flush a chipset cache.
To that end, we have i915_gem_chipset_flush() so use it.

Suggested-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170926153409.7928-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>