drm/i2c: tda998x: Choose between atomic or non atomic dpms helper

Choose between atomic or non atomic connector dpms helper. If tda998x
is connected to a drm driver that does not support atomic modeset
calling drm_atomic_helper_connector_dpms() causes a crash when the
connectors atomic state is not initialized. The patch implements a
driver specific connector dpms helper that calls
drm_atomic_helper_connector_dpms() if driver supports DRIVER_ATOMIC
and otherwise it calls the legacy drm_helper_connector_dpms().

Fixes commit 9736e988 ("drm/i2c: tda998x: Add support for atomic
modesetting").

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>