drm/ttm: move SG flag check into ttm_bo_vm_reserve

Just check earlier if a BO can be page faulted in the first place.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/392321/