drm/mgag200: Allocate instance of struct mga_i2c_chan dynamically

Allocate instances of struct mga_i2c_chan in mgag200_ddc_create()
and return a pointer to the contained i2c adapter. The callers of
the function are now independent from 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/20240513125620.6337-5-tzimmermann@suse.de