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

drm/ast: Store AST device in struct ast_ddc

The DDC code needs the AST device. Store a pointer in struct ast_ddc
and avoid internal upcasts. Improves type safety within the DDC code.

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/20240325200855.21150-9-tzimmermann@suse.de

drm/ast: Pass AST device to ast_ddc_create()

The DDC code needs the AST device. Pass it to ast_ddc_create() and
avoid an internal upcast. Improves type safety within the DDC code.

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/20240325200855.21150-8-tzimmermann@suse.de

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

drm/ast: Move DDC code to ast_ddc.{c,h}

Rename ast_i2c.c to ast_ddc.c and move its interface into the
new header ast_ddc.h. Update all include statements as necessary
and change the adapter name to 'AST DDC bus'.

This avoids including I2C headers in the driver's main header file,
which doesn't need them. Renaming files to _ddc indicates that the
code is about the DDC. I2C is really just the underlying bus here.

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/20240325200855.21150-6-tzimmermann@suse.de