drm/amdgpu: Delete cgs wrapper functions for gpu memory manager

delete those cgs interfaces:
amdgpu_cgs_alloc_gpu_mem
amdgpu_cgs_free_gpu_mem
amdgpu_cgs_gmap_gpu_mem
amdgpu_cgs_gunmap_gpu_mem
amdgpu_cgs_kmap_gpu_mem
amdgpu_cgs_kunmap_gpu_mem

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