drm/amdgpu/sdma4: use num_instances for clock/powergating config

Rather then relying on the asic type for the second instance.
Makes it more consistent with the rest of the code.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>