drm/mgag200: Inline mgag200_i2c_init()

The function mgag200_i2c_init() is an internal helper that sets
up the i2c data structure. Inline its code into the only caller.
Rearrange the individual steps to separate among i2c algorithm,
adapter and fields in struct mga_i2c_chan.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240513125620.6337-6-tzimmermann@suse.de