drm/ttm: add the missed global memory count update

when ttm_mem_global_alloc_page fails, we should call
ttm_mem_global_free_page to update memory count for
the ttm pages which already run ttm_mem_global_alloc_page
successfully

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