drm/ast: Allocate instance of struct ast_i2c_chan with managed helpers

Replace kzalloc() with drmm_kzalloc() and thereby put the release of
the I2C instance into a separate action. Avoids explicit error roll-
back in ast_i2c_chan_create(). 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/20240325200855.21150-5-tzimmermann@suse.de