drm/nouveau: by default use low bpp framebuffer on low memory cards

Framebuffer's BPP is not that important but can waste significant part
of memory on low-VRAM cards. Lower it to 8bpp on < 32MB cards and to
16bpp on 64MB cards. It can still be overridden by video= option.
Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_fbcon.c

@@ -487,6 +487,7 @@ int nouveau_fbcon_init(struct drm_device *dev)
 {
 	struct drm_nouveau_private *dev_priv = dev->dev_private;
 	struct nouveau_fbdev *nfbdev;
+	int preferred_bpp;
 	int ret;
 
 	nfbdev = kzalloc(sizeof(struct nouveau_fbdev), GFP_KERNEL);
@@ -505,7 +506,15 @@ int nouveau_fbcon_init(struct drm_device *dev)
 	}
 
 	drm_fb_helper_single_add_all_connectors(&nfbdev->helper);
-	drm_fb_helper_initial_config(&nfbdev->helper, 32);
+
+	if (dev_priv->vram_size <= 32 * 1024 * 1024)
+		preferred_bpp = 8;
+	else if (dev_priv->vram_size <= 64 * 1024 * 1024)
+		preferred_bpp = 16;
+	else
+		preferred_bpp = 32;
+
+	drm_fb_helper_initial_config(&nfbdev->helper, preferred_bpp);
 	return 0;
 }