drm/ttm: make global mutex and use count static

Only needed during device hot plug and remove and not exported.

Signed-off-by: Christian König <christian.koenig@amd.com>
Suggested-by: Bernard <bernard@vivo.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210409110730.2958-1-christian.koenig@amd.com