drm/ttm: remove pointers to globals

As the name says global memory and bo accounting is global. So it doesn't
make to much sense having pointers to global structures all around the code.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Thomas Hellström <thellstrom@vmware.com>
Link: https://patchwork.freedesktop.org/patch/332879/