drm/ttm: initialize globals during device init (v2)

Make sure that the global BO state is always correctly initialized.

This allows removing all the device code to initialize it.

v2: fix up vbox (Alex)

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>