drm/probe-helper: Do not fail from drmm_kms_helper_poll_init()

Failing to set up connector polling is not significant enough to
fail device probing. Print a warning and return nothing from the
init helper.

This only affects the managed init function. The unmanaged init
already never fails with an error.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250303145604.62962-4-tzimmermann@suse.de