drm/ast: Rename struct ast_i2c_chan to struct ast_ddc

The struct struct ast_i2c_chan represents the Display Data Channel
(DDC); I2C is the underlying bus. Rename the structure, the variables
and the helper ast_i2c_create() to ddc-like terms. No functional
changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240325200855.21150-7-tzimmermann@suse.de