drm: fix drm_drv_uses_atomic_modeset on non modesetting drivers.

vgem seems to oops on the intel CI due to the vgem debugfs init
hitting this path now.

Check if we have mode_config funcs before checking one.

Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180918062018.24942-1-airlied@gmail.com