st/mesa: purge framebuffers with current context after unbinding winsys buffers

With commit c89e8470, framebuffers are purged after unbinding context,
but this change also introduces a heap corruption when running Rhino application
on VMware svga device. Instead of purging the framebuffers after the context
is unbound, this patch first ubinds the winsys buffers, then purges the framebuffers
with the current context, and then finally unbinds the context.

This fixes heap corruption.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Brian Paul <brianp@vmware.com>
(cherry picked from commit b480adfa)