drm/exynos: don't use DMA_ERROR_CODE

DMA_ERROR_CODE already isn't a valid API to user for drivers and will
go away soon.  exynos_drm_fb_dma_addr uses it a an error return when
the passed in index is invalid, but the callers never check for it
but instead pass the address straight to the hardware.

Add a WARN_ON instead and just return 0.
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/gpu/drm/exynos/exynos_drm_fb.c

@@ -181,8 +181,8 @@ dma_addr_t exynos_drm_fb_dma_addr(struct drm_framebuffer *fb, int index)
 {
 	struct exynos_drm_fb *exynos_fb = to_exynos_fb(fb);
 
-	if (index >= MAX_FB_BUFFER)
-		return DMA_ERROR_CODE;
+	if (WARN_ON_ONCE(index >= MAX_FB_BUFFER))
+		return 0;
 
 	return exynos_fb->dma_addr[index];
 }