drm/amdgpu/vcn:Remove unused code

The following WREG32_SOC15_DPG_MODE will overwrite register
mmUVD_CGC_CTRL. This code can be removed.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>