drm/i915: Remove obsolete i915_gem_object_flush_active()

Since we track requests, and requests are always added to the GPU fully
formed, we never have to flush the incomplete request and know that the
given request will eventually complete without any further action on our
part.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470293567-10811-15-git-send-email-chris@chris-wilson.co.uk