Add support for dynamically setting the virtual display surface.

Previously, the surface that backs a virtual display had to be set
at the time when the display was created.  This change now makes
it possible to set or remove the surface later.  The virtual display
is treated as if it were "off" while no surface is attached to it.

Change-Id: Ib4fdbbb8b4ee79f0fb9ceb648f9bda4a8fa6a2ca