drm/radeon: skip async dma init on r6xx

The hw is buggy and it's not currently used, but it's
currently still initialized by the driver.  Skip the init.
Skipping init also seems to improve stability with dpm on
some r6xx asics.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=66963

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>