drm/amdgpu: add vce bypass mode for tonga.

fix issue that encode test failed on the second time when
vce dpm enabled on tonga.

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>