drm/udl: Use drmm_add_final_kfree

With this we can drop the final kfree from the release function.

v2: We need drm_dev_put to unroll the driver creation (once
drm_dev_init and drmm_add_final_kfree suceeded), otherwise
the drmm_ magic doesn't happen.

v3: Actually squash in the fixup (Laurent).

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Sean Paul <sean@poorly.run>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Emil Velikov <emil.l.velikov@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-7-daniel.vetter@ffwll.ch