drm/tegra: restrict plane loops to legacy planes

In Matt Ropers primary plane series a set of prep patches like

commit af2b653b
Author: Matt Roper <matthew.d.roper@intel.com>
Date:   Tue Apr 1 15:22:32 2014 -0700

    drm/i915: Restrict plane loops to only operate on overlay planes (v2)

ensured that all exisiting users of the mode_config->plane_list
wouldn't change behaviour. Unfortunately tegra seems to have fallen
through the cracks. Fix it.

This regression was introduced in

commit e13161af
Author: Matt Roper <matthew.d.roper@intel.com>
Date:   Tue Apr 1 15:22:38 2014 -0700

    drm: Add drm_crtc_init_with_planes() (v2)

The result was that we've unref'ed the fb for the primary plane twice,
leading to a use-after free bug. This is because the drm core will
already set crtc->primary->fb to NULL and do the unref for us, and the
crtc disable hook is called by the drm crtc helpers for exactly this
case.

Aside: Now that the fbdev helpers clean up planes there's no longer a
need to do this in drivers. So this could probably be nuked entirely
in linux-next.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>