drm/atomic: Use drm_drv_uses_atomic_modeset() for debugfs creation

As pointed out by Daniel Vetter, we should be usinng
drm_drv_uses_atomic_modeset() for determining whether or not we want to
make the debugfs nodes for atomic instead of checking DRIVER_ATOMIC, as
the former isn't an accurate representation of whether or not the driver
is actually using atomic modesetting internally (even though it might
not be exposing atomic capabilities).

Signed-off-by: Lyude Paul <lyude@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@vger.kernel.org
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180917173733.21293-1-lyude@redhat.com