drm/tegra: fb: Do not destroy framebuffer

Drop a reference instead of directly calling the framebuffer .destroy()
callback at fbdev free time. This is necessary to make sure the object
isn't destroyed if anyone else still has a reference.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>