drm: writeback: Create drmm variants for drm_writeback_connector initialization

To allows driver to only use drmm objects, add helper to create
drm_writeback_connectors with automated lifetime management.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20250116-google-vkms-managed-v9-7-3e4ae1bd05a0@bootlin.com


Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>