drm/ast: Automatically clean up poll helper

Automatically clean up the conncetor-poll thread as part of the DRM
device release. The new helper drmm_kms_helper_poll_init() provides
a shared implementation for all drivers.

v6:
- fix kernel doc comment (Sui, kernel test robot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Acked-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Link: https://patchwork.freedesktop.org/patch/msgid/20240325200855.21150-14-tzimmermann@suse.de