drm/ttm: add a simple assign mem to bo wrapper

This pattern is called in a few places, just clean it up.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200917043040.146575-6-airlied@gmail.com