drm/amdgpu: refine vce3.0 initialize.

1. disable vce cg when vce hw initialize.
2. initizlize vce clock to 10KHz fo dgpu,
   so no need to set bypass clock to vce.

Change-Id: I934c2c4820cc95c1bfa2fa41ff0f40a0d3cd1c40
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>