drm/tegra: Stop using drm_framebuffer_unregister_private

This is the deprecated function for when you embedded the framebuffer
somewhere else (which breaks refcounting). But Tegra is using
drm_framebuffer_remove and a free-standing FB, so this is redundant.

One caveat here is that the failure path in the init code still
manually cleaned up the fb. I presume that was an oversight and
changed it over to drm_framebuffer_remove too.

Cc: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1482835765-12044-5-git-send-email-daniel.vetter@ffwll.ch
Link: http://patchwork.freedesktop.org/patch/msgid/1482835765-12044-2-git-send-email-daniel.vetter@ffwll.ch