drm/ttm: Provide ttm_bo_global_{init/release}() for struct ttm_bo_global

So far, struct ttm_bo_global_ref was the only way of initializing a struct
ttm_bo_global. Providing separate initializer and release functions for
struct ttm_bo_global gives drivers the option of implementing their own
init and release callbacks for drm_global_references of type
DRM_GLOBAL_TTM_BO.

The original functions for initializing and releasing via struct
ttm_bo_global_ref are wrappers around the new interfaces.

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>