drm/vmwgfx: Fix Ubuntu 17.10 Wayland black screen issue

This is an extension of Commit 7c20d21 ("drm/vmwgfx: Work
around mode set failure in 2D VMs")

With Wayland desktop and atomic mode set, during the mode setting
process there is a moment when two framebuffer sized surfaces
are being pinned.  This was not an issue with Xorg.

Since this only happens during a mode change, there should be no
performance impact by increasing allowable mem_size.

Signed-off-by: Sinclair Yeh <syeh@vmware.com>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>

Cc: stable@vger.kernel.org

Author: Sinclair Yeh

drivers/gpu/drm/vmwgfx/vmwgfx_drv.c

@@ -721,7 +721,7 @@ static int vmw_driver_load(struct drm_device *dev, unsigned long chipset)
 		 * allocation taken by fbdev
 		 */
 		if (!(dev_priv->capabilities & SVGA_CAP_3D))
-			mem_size *= 2;
+			mem_size *= 3;
 
 		dev_priv->max_mob_pages = mem_size * 1024 / PAGE_SIZE;
 		dev_priv->prim_bb_mem =