As the GTT is outside of the powerwell, we can simplify flushing the
GGTT writes by using an unchecked mmio write and post.

v2: s/unc/uncore/
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/20190604120022.20472-3-chris@chris-wilson.co.uk