drm/ttm: Remove ttm_bo_reference and ttm_bo_unref

Both functions are obsolete and all calls have been replaced by
ttm_bo_get and ttm_bo_put.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>