intel: avoid unnecessary front buffer flushing/updating

intel: avoid unnecessary front buffer flushing/updating

Before, if we just called glXMakeCurrent() and didn't render anything we'd
still trigger a flushFrontBuffer() call.

Now only set the intel->front_buffer_dirty field at state validation time
just before we draw something.

NOTE: additional calls to intel_check_front_buffer_rendering() might be
needed if I missed some rendering paths.