drm/mgag200: Store pointer to struct mga_device in struct mga_i2c_chan

Avoid upcasting to struct mga_device in i2c code by storing the
pointer directly. 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/20240513125620.6337-4-tzimmermann@suse.de