r600g: don't use the CB/DB CP COHER logic on r6xx
r600g: don't use the CB/DB CP COHER logic on r6xx

There are hw bugs.  Flush and inv event is sufficient.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=66837

Signed-off-by: Alex Deucher <08dc22c6156113f2deff178e35e3ed9b24d6af9e@amd.com>

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