drm/amdgpu/cz: add code to enable forcing VCE clocks

VCE DPM works similarly to SCLK DPM.  Add a similar interface
for VCE for forcing the VCE clocks.

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