drm/amdgpu/vi: add missing error handling when setting uvd dclk

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reported-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>