sna: Compile fix for non-debug builds

sna_display.c: In function 'get_fb':
sna_display.c:181:3: warning: implicit declaration of function 'assert_scanout' [-Wimplicit-function-declaration]
sna_display.c:181:3: warning: nested extern declaration of 'assert_scanout' [-Wnested-externs]

* sigh

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/sna_display.c

@@ -162,6 +162,8 @@ static void assert_scanout(struct kgem *kgem, struct kgem_bo *bo,
 
 	assert(width == info.width && height == info.height);
 }
+#else
+#define assert_scanout(k, b, w, h)
 #endif
 
 static unsigned get_fb(struct sna *sna, struct kgem_bo *bo,