r600g: rework cache flushing

r600g: rework cache flushing

This also significantly improves the RV670 flush by using the CB1 flush
*always* and also DEST_BASE_0_ENA, which appears to magically fix some tests.
I am not entirely sure, but it's possible that RV670 flushing is fixed
completely.

v2: fix cayman by flushing texture cache instead of vertex cache

Thanks to Dave Airlie for testing Cayman.