drm/amdgpu: add sdma instance check for gfx11 CGCG

For some ASICs, like GFX IP v11.0.1, only have one SDMA instance,
so not need to configure SDMA1_RLC_CGCG_CTRL for this case.

Signed-off-by: Tim Huang <tim.huang@amd.com>
Reviewed-by: Yifan Zhang <yifan1.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>