drm/amdgpu: initialize GDS/GWS/OA domains even when they are zero sized

Stops crashing on SI.

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