intel: Throttle after doing copyregion/swapbuffers round trip

intel: Throttle after doing copyregion/swapbuffers round trip

Before we would throttle in the flush callback prior to round-tripping
to the server to do copyregion or swapbuffer.  Now, instead just note
that we need to throttle and do it in intel_prepare_render(), which
will be called after receiving the response from the server but before
we start rendering the next frame.  Even if the server also throttles
us in swapbuffer, this just makes the throttling a no-op when we hit
intel_prepare_render().  With that we can drop the
using_dri2_swapbuffers hack and just always throttle.