st/mesa: tell VBO module to always unmap buffers before drawing

Without this it's possible to wind up in a draw call with the
glBegin/End VBO still in a mapped state.  This is a problem for
the SVGA3D driver and probably not good for other HW drivers.

src/mesa/state_tracker/st_context.c

@@ -144,6 +144,10 @@ st_create_context_priv( struct gl_context *ctx, struct pipe_context *pipe )
    /* we want all vertex data to be placed in buffer objects */
    vbo_use_buffer_objects(ctx);
 
+
+   /* make sure that no VBOs are left mapped when we're drawing. */
+   vbo_always_unmap_buffers(ctx);
+
    /* Need these flags:
     */
    st->ctx->FragmentProgram._MaintainTexEnvProgram = GL_TRUE;