drm/ast: Rename struct ast_private to struct ast_device

The data structure struct ast_private represents an AST device. Its
name comes from the time when it was allocated and stored separately
in struct drm_device.dev_private. The DRM device is now embedded, so
rename struct ast_private to struct ast_device.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230221155745.27484-4-tzimmermann@suse.de