drm/tegra: Remove duplicate framebuffer num_planes

drm_framebuffer already stores num_planes for us.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Thierry Reding <treding@nvidia.com>