drm/i915: Fix semaphore emission for BDW+ RCS ringbuffer emission

The required number of dwords for semaphore emission on BDW RCS is 8,
not 6 - leading to ring buffer corruption and immediate GPU hangs when
using ringbuffer submission.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170324151724.32640-2-chris@chris-wilson.co.ukReviewed-by: Michał Winiarski <michal.winiarski@intel.com>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -2117,7 +2117,7 @@ int intel_init_render_ring_buffer(struct intel_engine_cs *engine)
 
 			num_rings =
 				hweight32(INTEL_INFO(dev_priv)->ring_mask) - 1;
-			engine->emit_breadcrumb_sz += num_rings * 6;
+			engine->emit_breadcrumb_sz += num_rings * 8;
 		}
 	} else if (INTEL_GEN(dev_priv) >= 6) {
 		engine->init_context = intel_rcs_ctx_init;