drm/i915: Discard some redundant cache domain flushes

Since commit a679f58d ("drm/i915: Flush pages on acquisition"), we
flush objects on acquire their pages and as such when we create an
object for the purpose of writing into it, we do not need to manually
flush.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614111053.25615-1-chris@chris-wilson.co.uk