drm/radeon: activate UVD clocks before sending the destroy msg

Make sure the UVD clocks are still active before sending
the destroy message, otherwise the hw might hang.

Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>