drm/amdgpu: don't init GDS pool if GDS size is 0 (v2)

SI cards don't expose GDS as a separate pool.  The CP manages
GDS and the UMDs use special CP packets to allocate GDS memory.

v2: drop extra whitespace change

bug: https://bugzilla.kernel.org/show_bug.cgi?id=194867



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