r600g: Don't set the dest cache bits on surface sync for R600_CONTEXT_FLUSH_AND_INV

r600g: Don't set the dest cache bits on surface sync for R600_CONTEXT_FLUSH_AND_INV

We are already emitting a EVENT_TYPE_CACHE_FLUSH_AND_INV_EVENT packet
when this flush flag is set, so flushing the dest caches with a
SURFACE_SYNC should not be necessary.

The motivation for this change is that emitting a SURFACE_SYNC packet with
the CB bits set was causing compute shaders to hang on Cayman.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>