drm/mgag200: Rename mgag200_i2c.c to mgag200_ddc.c

Rename the source file according to its content. 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/20240513125620.6337-8-tzimmermann@suse.de