i965/blorp: Do and end-of-pipe sync on both sides of fast-clear ops

i965/blorp: Do and end-of-pipe sync on both sides of fast-clear ops

We've discovered in the Vulkan driver that simply doing the end-of-pipe
sync afterwards is insufficient.  The specific requirement stated in the
PRM is that you have to do one every time you transition between the
tree modes of "clear", "render", and "resolve".  This is GL, so we could
track it but any attempt to do so would most likely get it wrong.  For
now, it's easier to just assume that every fast-clear op is an island
and do the sync both before and after.

This also removes the unneeded flush and stall after slow-clear
operations.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Cc: "17.0 17.1" <mesa-stable@lists.freedesktop.org>