drm/mgag200: Fail on I2C initialization errors

Initialization of the I2C adapter was allowed to fail. The mgag200
driver would have continued without DDC support. Had this happened in
practice, it would have led to segmentation faults in the connector
code. Resolve this problem by failing driver initialization on I2C-
related errors.

v2:
	* initialize 'ret' before drm_err() (kernel test robot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220516134343.6085-3-tzimmermann@suse.de