drm/exynos: return return value of exynos_crtc->enable_vblank

Instead of blindly ignore the return value of enable_vblank return it
to the upper DRM layer for error handling.

Suggested-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_crtc.c

@@ -177,7 +177,7 @@ int exynos_drm_crtc_enable_vblank(struct drm_device *dev, int pipe)
 		return -EPERM;
 
 	if (exynos_crtc->ops->enable_vblank)
-		exynos_crtc->ops->enable_vblank(exynos_crtc);
+		return exynos_crtc->ops->enable_vblank(exynos_crtc);
 
 	return 0;
 }