drm/amdgpu: refine code for VCE2.0 and related dpm code.

v2: clean up vce cg function.
    use sw cg when vce stoped.

1. implement vce_stop function.
2. not start vce when hw_init.
3. refine vce cg/pg code.
4. delete bypass mode.

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