drm/ttm: Force re-init if ttm_global_init() fails

If we have a failure, decrement the reference count so that the next
call to ttm_global_init() will actually do something instead of assume
everything is all set up.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Fixes: 62b53b37 ("drm/ttm: use a static ttm_bo_global instance")
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210720181357.2760720-5-jason@jlekstrand.net


Signed-off-by: Christian König <christian.koenig@amd.com>