drm/omap: page_flip: return -EBUSY if flip pending

The DRM documentation says:

"If a page flip is already pending, the page_flip operation must return
-EBUSY."

Currently omapdrm returns -EINVAL instead. Fix omapdrm by returning
-EBUSY.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>

drivers/gpu/drm/omapdrm/omap_crtc.c

@@ -606,7 +606,7 @@ static int omap_crtc_page_flip_locked(struct drm_crtc *crtc,
 	if (omap_crtc->old_fb) {
 		spin_unlock_irqrestore(&dev->event_lock, flags);
 		dev_err(dev->dev, "already a pending flip\n");
-		return -EINVAL;
+		return -EBUSY;
 	}
 
 	omap_crtc->event = event;