drm/amdgpu: simplify the sdma 4_x MGCG/MGLS logic.

SDMA 4_x asics share the same MGCG/MGLS setting.

Signed-off-by: Feifei Xu <Feifei.Xu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>