Use ttm_bo_mem_space instead of manually allocating GART space.

This allows us to evict BOs when there isn't enought GART space any more.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>