drm/amd/pm: add inst to dpm_set_powergating_by_smu

Add an instance parameter to amdgpu_dpm_set_powergating_by_smu() function,
and use the instance to call set_powergating_by_smu().

v2: remove duplicated functions.

remove for-loop in amdgpu_dpm_set_powergating_by_smu(), and temporarily
move it to amdgpu_dpm_enable_vcn(), in order to keep the exact same logic
as before, until further separation in next patch.

v3: drop SI logic in amdgpu_dpm_enable_vcn().

Signed-off-by: Boyuan Zhang <boyuan.zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>