intel: Flag NEW_BUFFERS when changing draw buffers.

There were entries to this function (most imporantly, prepare_render
-> update_renderbuffers) that wouldn't have had NEW_BUFFERS set, but
brw_wm_surface_state (the i965 state tracking the drawing regions)
expected this to change.

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

@@ -231,6 +231,7 @@ intel_draw_buffer(GLcontext * ctx, struct gl_framebuffer *fb)
 
    intel->vtbl.set_draw_region(intel, colorRegions, depthRegion, 
                                fb->_NumColorDrawBuffers);
+   intel->NewGLState |= _NEW_BUFFERS;
 
    /* update viewport since it depends on window size */
 #ifdef I915