drm/mgag200: on cards with < 2MB VRAM default to 16-bit

This aligns with what the userspace -mga driver does in
the same situation.
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/mgag200/mgag200_fb.c

@@ -282,6 +282,11 @@ int mgag200_fbdev_init(struct mga_device *mdev)
 {
 	struct mga_fbdev *mfbdev;
 	int ret;
+	int bpp_sel = 32;
+
+	/* prefer 16bpp on low end gpus with limited VRAM */
+	if (IS_G200_SE(mdev) && mdev->mc.vram_size < (2048*1024))
+		bpp_sel = 16;
 
 	mfbdev = devm_kzalloc(mdev->dev->dev, sizeof(struct mga_fbdev), GFP_KERNEL);
 	if (!mfbdev)
@@ -301,7 +306,7 @@ int mgag200_fbdev_init(struct mga_device *mdev)
 	/* disable all the possible outputs/crtcs before entering KMS mode */
 	drm_helper_disable_unused_functions(mdev->dev);
 
-	drm_fb_helper_initial_config(&mfbdev->helper, 32);
+	drm_fb_helper_initial_config(&mfbdev->helper, bpp_sel);
 
 	return 0;
 }

drivers/gpu/drm/mgag200/mgag200_main.c

@@ -217,6 +217,9 @@ int mgag200_driver_load(struct drm_device *dev, unsigned long flags)
 
 	drm_mode_config_init(dev);
 	dev->mode_config.funcs = (void *)&mga_mode_funcs;
+	if (IS_G200_SE(mdev) && mdev->mc.vram_size < (2048*1024))
+		dev->mode_config.preferred_depth = 16;
+	else
 		dev->mode_config.preferred_depth = 24;
 	dev->mode_config.prefer_shadow = 1;