drm/tegra: Obtain head number from DT

The head number of a given display controller is fixed in hardware and
required to program outputs appropriately. Relying on the driver probe
order to determine this number will not work, since that could yield a
situation where the second head was probed first and would be assigned
head number 0 instead of 1.

By explicitly specifying the head number in the device tree, it is no
longer necessary to rely on these assumptions. As a fallback, if the
property isn't available, derive the head number from the display
controller node's position in the device tree. That's somewhat more
reliable than the previous default but not a proper solution.

Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>