i915: Fix leak of ViewportMatrix data on context destroy.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=30217

src/mesa/drivers/dri/intel/intel_context.c

@@ -922,6 +922,8 @@ intelDestroyContext(__DRIcontext * driContextPriv)
       /* free the Mesa context */
       _mesa_free_context_data(&intel->ctx);
 
+      _math_matrix_dtr(&intel->ViewportMatrix);
+
       FREE(intel);
       driContextPriv->driverPrivate = NULL;
    }