drm/amdgpu/raven: power up/down VCN via the SMU (v2)

By default VCN is powered down like SDMA, power it up/down
on driver load/unload.

[Rui: Fix to add the parameter 0 to un-gate VCN] v2

Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>