drm/ttm: Rename ttm_bo_global_{init,release}() to ttm_bo_global_ref_{,}()

The functions ttm_bo_global_init() and ttm_bo_global_release() do not
receive an argument of type struct ttm_bo_global. Both take a struct
drm_global_reference that contains points to a struct ttm_bo_global_ref.
Renaming them reflects this.

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