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 --git a/drivers/gpu/drm/tegra/drm.h b/drivers/gpu/drm/tegra/drm.h
index 266aae0..bb8b130 100644
--- a/drivers/gpu/drm/tegra/drm.h
+++ b/drivers/gpu/drm/tegra/drm.h
@@ -186,6 +186,7 @@
 	const struct tegra_output_ops *ops;
 	enum tegra_output_type type;
 
+	struct drm_panel *panel;
 	struct i2c_adapter *ddc;
 	const struct edid *edid;
 	unsigned int hpd_irq;