Drivers that care about crtc positions on the screen to ensure that vblank
works correctly need to be notified when crtcs are changed.

Provide a hook in the mode setting code that is invoked whenever any
configuration is done to the screen.

Use this new hook in the DRI code so that DRI clients are notified and
receive updated information.

Signed-off-by: Keith Packard <keithp@keithp.com>