gma500: wide framebuffer memory

If we set a small text framebuffer and have a bigger scanout then we want
to send black not random bits for the overscan.
Signed-off-by: NAlan Cox <alan@linux.intel.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>

drivers/gpu/drm/gma500/framebuffer.c

@@ -408,6 +408,8 @@ static int psbfb_create(struct psb_fbdev *fbdev,
 			return -ENOMEM;
 	}
 
+	memset(dev_priv->vram_addr + backing->offset, 0, size);
+
 	mutex_lock(&dev->struct_mutex);
 
 	info = framebuffer_alloc(0, device);