drm/ttm: Remove reference to the mem_glob member

It was removed in:

Author: Christian König <christian.koenig@amd.com>
Date:   Wed Sep 25 11:38:50 2019 +0200

    drm/ttm: remove pointers to globals

Signed-off-by: Maya Rashish <coypu@sdf.org>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/360750/
Signed-off-by: Christian König <christian.koenig@amd.com>