r600g: don't emit surface_sync after FLUSH_AND_INV_EVENT

r600g: don't emit surface_sync after FLUSH_AND_INV_EVENT

It shouldn't be needed since the FLUSH_AND_INV_EVENT has already
made sure the destination caches are flushed.  Additionally,
we didn't previously emit the surface_sync until this commit:
http://cgit.freedesktop.org/mesa/mesa/commit/?id=e5e4c07e7964a3258ed02b530bcdc24c0650204b
Emitting them together causes hangs in compute on cayman/TN
and hangs in Heaven on evergreen.

Note: this patch is a candidate for the 9.1 branch, but requires:
http://cgit.freedesktop.org/mesa/mesa/commit/?id=156bcca62c9f4e79e78929f72bc085757f36a65a
as well.

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>