drm/amdgpu: remove power on/off SDMA in SMU hw_init/fini()

Currently, we don't find some neccesities to power on/off
SDMA in SMU hw_init/fini(). It makes more sense in SDMA
hw_init/fini().

Signed-off-by: Lang Yu <lang.yu@amd.com>
Reviewed-by: Kevin Wang <kevinyang.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>