drm/mcde: Use drmm_add_final_kfree

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

v2: Fix unused variable warning, spotted while applying.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-13-daniel.vetter@ffwll.ch