drm/ttm: fix deref of bo->ttm without holding the lock v2

We need to grab the resv lock first before doing that check.

v2 (chk): simplify the change for -fixes

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210528130041.1683-1-christian.koenig@amd.com