ARM: tegra: Add head numbers to display controllers

The number of the head specifies the index of the display controller
unit and is required to properly configure outputs so that they receive
video data from the correct source.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>