drm/amdgpu: Add comment to function amdgpu_ttm_alloc_gart

Add comments to refect what function does

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