drm/ast: Define struct ast_ddc in ast_ddc.c

Move the definition of struct ast_ddc to ast_ddc.c and return the i2c
adapter from ast_ddc_create(). Update callers accordingly. Avoids
including Linux i2c header files, except where required. 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/20240403103325.30457-4-tzimmermann@suse.de