drm/i915: Use unchecked uncore writes to flush the GTT

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