drm/amdgpu: fix bug mclk can't change on Polaris

the root cause is we gate the clock to uvd vcpu.
mclk's change should need the response from uvd if
it is power on.

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>