drm/panel: otm8009a: Fix glitches by moving backlight enable to otm8009a_enable()

The backlight 1st update was in the otm8009a_prepare() function for a
bad reason: backlight was not working in video mode and the
otm8009a_prepare() is in command mode for the init sequence. As the
backlight is now fixed (no low-power mode), it is good to put it back
in the otm8009a_enable() function, avoiding also image glitches visible
on some "slow" devices.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423141054.13128-3-philippe.cornu@st.com

drivers/gpu/drm/panel/panel-orisetech-otm8009a.c

@@ -330,13 +330,6 @@ static int otm8009a_prepare(struct drm_panel *panel)
 
 	ctx->prepared = true;
 
-	/*
-	 * Power on the backlight. Note: end-user still controls brightness
-	 * Note: ctx->prepared must be true before updating the backlight.
-	 */
-	ctx->bl_dev->props.power = FB_BLANK_UNBLANK;
-	backlight_update_status(ctx->bl_dev);
-
 	return 0;
 }
 
@@ -344,6 +337,16 @@ static int otm8009a_enable(struct drm_panel *panel)
 {
 	struct otm8009a *ctx = panel_to_otm8009a(panel);
 
+	if (ctx->enabled)
+		return 0;
+
+	/*
+	 * Power on the backlight. Note: end-user still controls brightness
+	 * Note: ctx->prepared must be true before updating the backlight.
+	 */
+	ctx->bl_dev->props.power = FB_BLANK_UNBLANK;
+	backlight_update_status(ctx->bl_dev);
+
 	ctx->enabled = true;
 
 	return 0;