drm/i2c: tda998x: Add support for atomic modesetting

When used with a DRIVER_ATOMIC enabled CRTC driver, the tda998x
will cause crashes due to missing atomic operations. Fill the
drm_connector_funcs struct with the atomic versions of the required
functions and add the atomic modeset specific callbacks.

Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>