drm/amd/pm: add inst to set_powergating_by_smu

Add an instance parameter to set_powergating_by_smu() function, and
re-write all amd_pm functions accordingly. Then use the instance to
call smu_dpm_set_vcn_enable().

v2: remove duplicated functions.

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

v3: add instance number in error message.

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>