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 88fea85f09
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>

src/mesa/drivers/dri/i965/brw_vec4.cpp

@@ -822,11 +822,11 @@ vec4_visitor::is_dep_ctrl_unsafe(const vec4_instruction *inst)
    (reg.type == BRW_REGISTER_TYPE_UD || \
     reg.type == BRW_REGISTER_TYPE_D)
 
-   /* From the destination hazard section of the spec:
-    * > Instructions other than send, may use this control as long as operations
-    * > that have different pipeline latencies are not mixed.
+   /* "When source or destination datatype is 64b or operation is integer DWord
+    * multiply, DepCtrl must not be used."
+    * May apply to future SoCs as well.
     */
-   if (brw->gen >= 8) {
+   if (brw->is_cherryview) {
       if (inst->opcode == BRW_OPCODE_MUL &&
          IS_DWORD(inst->src[0]) &&
          IS_DWORD(inst->src[1]))