drm/cirrus: Use drmm_add_final_kfree

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

I also noticed that cirrus forgot to call drm_dev_fini().

v2: Don't call kfree(cirrus) after we've handed overship of that to
drm_device and the drmm_ stuff.

Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: virtualization@lists.linux-foundation.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-10-daniel.vetter@ffwll.ch