drm/udl: Unmap buffer object after damage update

Udl keeps a BO mapped for its entire lifetime if it has been used in a
damage update at least once. The BO's free callback release the mapping
before it frees the BO.

Change this behaviour to unmap immediately after the damage update, so
SHMEM's implementation of free can be used.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191114141025.32198-2-tzimmermann@suse.de