drm/radeon/kms/r7xx: add workaround for hw issue with HDP flush

Use of HDP_*_COHERENCY_FLUSH_CNTL can cause a hang in certain
situations.  Add workaround.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>