drm/udl: Convert to atomic-modesetting helpers

Replace simple-KMS helpers with regular atomic-modesetting helpers.
The simple-KMS helpers introduce a mid-layer abstraction without
added functionality. Using regular atomic helpers makes the driver's
implementation more discoverable and simplifies code sharing.

The conversion effectively open-codes the simple-KMS functions and
data structure within udl. No functional changes.

v2:
	* don't use the atomic_disable plane helper

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006095355.23579-8-tzimmermann@suse.de