drm/amdgpu: only send one sdma firmware for sienna_cichlid

As all four sdma firmware are same, PSP only receive one SDMA fw.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>