drm/amd/pm: fulfill the support for SMU13 `pp_dpm_dcefclk` interface

Fulfill the incomplete SMU13 `pp_dpm_dcefclk` implementation.

Reported-by: Guan Yu <guan.yu@amd.com>
Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>