drm/ttm:fix wrong decoding of bo_count

we observe abnormal number from:
/sys/devices/virtual/drm/amdttm/buffer_objects/bo_count

bo_count is atomic_inc which is "int" type,
shouldn't explicitly turn it to unsigned long.

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