drm/fsl-dcu: fix opencoded use of drm_panel_*

Use drm_panel_get_modes() to access modes.
This has a nice side effect to simplify the code.

drm_panel_get_modes() may return a negative value if
for example panel is NULL. This is a small change
compared to before, but really what we want.

v2:
- Add more info to changelog (Stefan)

Signed-off-by: Sam Ravnborg <[e-mail]>
Acked-by: Stefan Agner <[e-mail]>
Reviewed-by: Emil Velikov <[e-mail]>
Cc: Alison Wang <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_rgb.c

···
 static int fsl_dcu_drm_connector_get_modes(struct drm_connector *connector)
 {
 	struct fsl_dcu_drm_connector *fsl_connector;
-	int (*get_modes)(struct drm_panel *panel);
-	int num_modes = 0;
 
 	fsl_connector = to_fsl_dcu_connector(connector);
-	if (fsl_connector->panel && fsl_connector->panel->funcs &&
-	    fsl_connector->panel->funcs->get_modes) {
-		get_modes = fsl_connector->panel->funcs->get_modes;
-		num_modes = get_modes(fsl_connector->panel);
-	}
-
-	return num_modes;
+	return drm_panel_get_modes(fsl_connector->panel);
 }
 
 static int fsl_dcu_drm_connector_mode_valid(struct drm_connector *connector,