drm/mgag200: Replace struct mga_i2c_chan with struct mgag200_ddc

Rename struct mga_i2c_chan to struct mgag200_ddc, define it in the
source file mgag200_i2c.c, and reorder its fields. Rename all related
variables from i2c to ddc. Also rename the i2c adapter accordingly.

Using the term 'ddc' documents the purpose of the code clearly. The
old term 'i2c' could refer to any functionality on an i2c bus. No
functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240513125620.6337-7-tzimmermann@suse.de