drm/i915: Remove superfluous posting reads after clear GGTT

The barrier here is not required - we apply the barrier before the range
is ever reused by the GPU instead.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-16-chris@chris-wilson.co.uk