drm/tegra: Implement panel support

Use the DRM panel framework to attach a panel to an output. If the panel
attached to a connector supports supports the backlight brightness
accessors, a property will be available to allow the brightness to be
modified from userspace.

Signed-off-by: Thierry Reding <treding@nvidia.com>

diff --cc Documentation/devicetree/bindings/gpu/nvidia,tegra20-host1x.txt
Simple merge

diff --cc drivers/gpu/drm/tegra/Kconfig
Simple merge

diff --cc drivers/gpu/drm/tegra/drm.h
Simple merge

diff --cc drivers/gpu/drm/tegra/output.c
Simple merge