drm/amdgpu: refine uvd_4.2 clock gate sequence.

1. partial revert commit 91db308d6e96.
   not set uvd bypass mode.
2. enable uvd cg before initialize uvd.
3. set uvd clock to default value 100MHz.

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>