drm/mgag200: Store positions of I2C data and clock bits in device info

The bits for accessing I2C data and clock channels varies among
models. Store them in the device-info structure for consumption
by the DDC code.

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/20220601112522.5774-11-tzimmermann@suse.de