drm/msm: add calls to prepare and unprepare panel

Prepare the panel before it's enabled and un-prepare after disable, this
will make sure that the regulators are switched on and off correctly.

Tested it on APQ8064 based IFC6410 with panel.
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/mdp/mdp4/mdp4_lcdc_encoder.c

@@ -346,8 +346,10 @@ static void mdp4_lcdc_encoder_disable(struct drm_encoder *encoder)
 
 	mdp4_write(mdp4_kms, REG_MDP4_LCDC_ENABLE, 0);
 
-	if (panel)
+	if (panel) {
 		drm_panel_disable(panel);
+		drm_panel_unprepare(panel);
+	}
 
 	/*
 	 * Wait for a vsync so we know the ENABLE=0 latched before
@@ -412,8 +414,10 @@ static void mdp4_lcdc_encoder_enable(struct drm_encoder *encoder)
 	if (ret)
 		dev_err(dev->dev, "failed to enable lcdc_clk: %d\n", ret);
 
-	if (panel)
+	if (panel) {
+		drm_panel_prepare(panel);
 		drm_panel_enable(panel);
+	}
 
 	setup_phy(encoder);