i965/fs: Don't let the EOT send message interfere with the MRF hack

i965/fs: Don't let the EOT send message interfere with the MRF hack

Previously, we just put the message for the EOT send as high in the file as
it would go.  This is because the register pre-filling hardware will stop
all over the early registers in the file in preparation for the next thread
while you're still sending the last message.  However, if something happens
to spill, then the MRF hack interferes with the EOT send message and, if
things aren't scheduled nicely, will stomp on it.

Cc: "10.5 10.6" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90520
Reviewed-by: Neil Roberts <neil@linux.intel.com>