drm/amdgpu/atom: add SetDCEClock helper

New cmd table for ELM/BAF for setting the dispclock or
dprefclock.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>