drm/vkms: Switch to managed for writeback connector

The current VKMS driver uses non-managed function to create
writeback connectors. It is not an issue yet, but in order
to support multiple devices easily, convert this code to
use drm and device managed helpers.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250116-google-vkms-managed-v9-8-3e4ae1bd05a0@bootlin.com


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