drm/ttm: use a static ttm_mem_global instance

As the name says we only need one global instance of ttm_mem_global.

Drop all the driver initialization and just use a single exported
instance which is initialized during BO global initialization.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>