drm/amd/amdgpu: don't set clockgating in uvd_v6_0_start()

This is handled properly by both DPM and PP externally.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>