egl_dri2: Fix one context, multiple surfaces.

egl_dri2: Fix one context, multiple surfaces.

When a context was made current to another surface, the old code did
this

  dri2_dpy->core->bindContext(cctx, ddraw, rdraw);
  dri2_dpy->core->unbindContext(old_cctx);

and there will be no current context due to the second line.

unbindContext should be called only when bindContext is not.  This fixes
a regression since d19afc57.  Thanks to Neil Roberts for noticing the
issue and creating a test case.