trace: fix incorrect trace_surface::base.context pointer

trace: fix incorrect trace_surface::base.context pointer

When a trace_surface object is created in trace_surf_create() we
weren't correctly setting the surface's context pointer.  Instead of
it being the trace context, it was the wrapped driver's context.
This caused things to blow up sometimes during surface deallocation.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>