drm/vmwgfx: Don't create proxy surface for cursor

With atomic, the cursor surface is treated like a FB.  Creating
a proxy surface for cursor doesn't gain us much benefit.

This fixes the issue on atomic enabled 2D VMs where the cursor
disappears.

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