drm/i915: drop the double-OP_STOREDW usage in blt_ring_flush

This has been introduced in "drm/i915: TLB invalidation with
MI_FLUSH_DW requires a post-sync op".

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>