drm/amdgpu: Enable SDMA context switching for CIK

Enable SDMA context switching on CIK (copied from sdma_v3_0.c).

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>