gldisplay: always unref the context on a collision

Otherwise we will leak GstGLContext's when adding the same context more than
once.

Fixes a regression caused by 5f9d10f603 in the
gstglcontext unit test that failed with:

Assertion 'tmp == NULL' failed

gst-libs/gst/gl/gstgldisplay.c

@@ -558,7 +558,6 @@ gst_gl_display_add_context (GstGLDisplay * display, GstGLContext * context)
     if (context == collision) {
       GST_LOG_OBJECT (display, "Attempting to add the same GL context %"
           GST_PTR_FORMAT ". Ignoring", context);
-      collision = NULL;
       ret = TRUE;
       goto out;
     }