drm/panel: Provide convenience wrapper for .get_modes()

Add a convenience wrapper for the struct drm_panel_funcs' .get_modes()
function so that not every driver needs to check that the panel driver
implements the function before calling it.
Signed-off-by: Ajay Kumar <ajaykumar.rs@samsung.com>
[treding: extract from larger patch, commit message]
Signed-off-by: Thierry Reding <treding@nvidia.com>

include/drm/drm_panel.h

@@ -112,6 +112,14 @@ static inline int drm_panel_enable(struct drm_panel *panel)
 	return panel ? -ENOSYS : -EINVAL;
 }
 
+static inline int drm_panel_get_modes(struct drm_panel *panel)
+{
+	if (panel && panel->funcs && panel->funcs->get_modes)
+		return panel->funcs->get_modes(panel);
+
+	return panel ? -ENOSYS : -EINVAL;
+}
+
 void drm_panel_init(struct drm_panel *panel);
 
 int drm_panel_add(struct drm_panel *panel);