drm: mxsfb: Implement drm_panel handling

Currently when the 'power-supply' regulator is passed via device tree
it does not actually work since drm_panel_prepare()/drm_panel_enable()
are never called.

Quoting Thierry Reding: "It should really call drm_panel_prepare() and
drm_panel_enable() while switching on the display pipeline and
drm_panel_disable(), followed by drm_panel_unprepare() while switching
off the display pipeline."

So do as suggested, so that the 'power-supply' regulator can be functional.
Reported-by: Breno Lima <breno.lima@nxp.com>
Suggested-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/mxsfb/mxsfb_drv.c

@@ -102,14 +102,18 @@ static void mxsfb_pipe_enable(struct drm_simple_display_pipe *pipe,
 {
 	struct mxsfb_drm_private *mxsfb = drm_pipe_to_mxsfb_drm_private(pipe);
 
+	drm_panel_prepare(mxsfb->panel);
 	mxsfb_crtc_enable(mxsfb);
+	drm_panel_enable(mxsfb->panel);
 }
 
 static void mxsfb_pipe_disable(struct drm_simple_display_pipe *pipe)
 {
 	struct mxsfb_drm_private *mxsfb = drm_pipe_to_mxsfb_drm_private(pipe);
 
+	drm_panel_disable(mxsfb->panel);
 	mxsfb_crtc_disable(mxsfb);
+	drm_panel_unprepare(mxsfb->panel);
 }
 
 static void mxsfb_pipe_update(struct drm_simple_display_pipe *pipe,