video: tegra: Move to using simple-panel and pwm-backlight

We have standard drivers for panels and backlights which can do most of the
work for us. Move the tegra20 LCD driver over to use those instead of custom
code.

This patch includes device tree changes for the nvidia boards. I have only
been able to test seaboard. If this patch is applied, these boards will
also need to be synced with the kernel, and updated to use display-timings:

   - colibri
   - medcom-wide
   - paz00
   - tec

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>