svga: fix vgpu10 query fencing

We don't want to flush the command buffer or sync on the fence when ending
a query (that kind of defeats the whole purpose of async queries).  Do that
instead in get_query_result().

Tested with Piglit, arbocclude, Sauerbraten game, Nobel Clinician Viewer,
ETQW.

Reviewed-by: Charmaine Lee <charmainel@vmware.com>