sdma is powered down by default in vbios,
need to power up in driver init.  Power it down
again on driver tear down.
Signed-off-by: NChunming Zhou <David1.Zhou@amd.com>
Reviewed-by: NHawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>