Support SDMA soft reset for SDMA v6.

V3: use ib test to check soft reset.
V4: squash in unused variable fix (Alex)
Signed-off-by: NLikun Gao <Likun.Gao@amd.com>
Reviewed-by: NHawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>