drm/vmwgfx: Fix Ubuntu 17.10 Wayland black screen issue

This is an extension of Commit 7c20d213 ("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