drm/tegra: Add support for panel prepare and unprepare routines

Modify tegra output driver to support the new panel calls:
prepare and unprepare.
Signed-off-by: NAjay Kumar <ajaykumar.rs@samsung.com>
Signed-off-by: NThierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/output.c

@@ -140,7 +140,9 @@ static void tegra_encoder_dpms(struct drm_encoder *encoder, int mode)
 	if (mode != DRM_MODE_DPMS_ON) {
 		drm_panel_disable(panel);
 		tegra_output_disable(output);
+		drm_panel_unprepare(panel);
 	} else {
+		drm_panel_prepare(panel);
 		tegra_output_enable(output);
 		drm_panel_enable(panel);
 	}