drm/qxl: Use drmm_add_final_kfree

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

Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: virtualization@lists.linux-foundation.org
Cc: spice-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20200323144950.3018436-8-daniel.vetter@ffwll.ch