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: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>