drm/ast: Handle failed I2C initialization gracefully

I2C initialization is allowed to fail. In this case, create a connector
without DDC adapter. The current code would dereference a NULL pointer.

Reading the modes from the connector is supposed to work without I2C
adapter. Add the respective test.

v2:
	* init edid to NULL to avoid uninitialized read (Dan)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20211206091125.29501-2-tzimmermann@suse.de