i965/vec4: Correct MUL destination hazard

i965/vec4: Correct MUL destination hazard

As it turns out, we were over-thinking the cause of the hang on
Cherryview. It's simply errata for Cherryview.

commit 88fea85f09e2252035bec66ab26c375b45b000f5
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Fri Nov 21 10:47:41 2014 -0800

    i965/vec4/gen8: Handle the MUL dest hazard exception

This is an explanation to why we never saw the hang on BDW.

NOTE: The problem the original patch was trying to fix does still exist. It will
have to be fixed at some point.

v2: Modify commit message, s/CHV/BDW

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=84212
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>