drm/amdgpu/powerplay: add an implementation for get_vce_clock_state (v3)

Used by the powerplay dpm code.

v2: update to the new API
v3: drop old include

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