drm/i915: Poison the request before emitting commands

If we poison the request before we emit commands, it should be easier to
spot when we execute an uninitialised request.

References: https://bugs.freedesktop.org/show_bug.cgi?id=100144
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170423170619.7156-2-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1694,6 +1694,7 @@ u32 *intel_ring_begin(struct drm_i915_gem_request *req, int num_dwords)
 
 	GEM_BUG_ON(ring->emit > ring->size - bytes);
 	cs = ring->vaddr + ring->emit;
+	GEM_DEBUG_EXEC(memset(cs, POISON_INUSE, bytes));
 	ring->emit += bytes;
 	ring->space -= bytes;
 	GEM_BUG_ON(ring->space < 0);