Running the complete deqp_gles2 seems to trigger an overflow in lrz
cmdstream.  We skip the blit clear fast-path if there have been any
draws (so mid-batch clears of any attached buffer hit the 3d pipe).
Which means it is safe to simply discard any lrz clear rendering.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <mesa/mesa!3569>