drm/i915: No need to wait for idle on L3 remap

As the L3 remapping is applied before the next execution, there is no
need to wait until all previous uses are idle, the application will not
occur any sooner.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466776558-21516-5-git-send-email-chris@chris-wilson.co.uk