R9270X PCIE 2.0 2 gig vram 4 gig ram.

Unreal demo Elemental only recently started working with git llvm/mesa, do no
bisection done on these.

The demo even when working is very broken - constant 1/4 sec stutters with a
couple of 1/2 - 1 sec.

But after -

commit 4439d469706699b4e69ef410ebc9115339f6e9e6
Author: Michel Dänzer <michel.daenzer@amd.com>
Date:   Thu Jul 31 18:43:49 2014 +0900

    drm/radeon: Always flush the HDP cache before submitting a CS to the GPU

    This ensures the GPU sees all previous CPU writes to VRAM, which makes it
    safe:

    * For userspace to stream data from CPU to GPU via VRAM instead of GTT
    * For IBs to be stored in VRAM instead of GTT
    * For ring buffers to be stored in VRAM instead of GTT, if the HPD flush
      is performed via MMIO


There are far longer pauses of many seconds - run time to unreal logo on "good"
is 3:45 on bad best 5:39 can be longer.

Screens to be attached show quite different vram/gtt usage between good and
bad.

Related to 

https://bugs.freedesktop.org/show_bug.cgi?id=82050

I bisected to the same commit in that bug, but applying the mesa patch

https://bugs.freedesktop.org/show_bug.cgi?id=82050#c19

doesn't help here.

CONFIG_CMA is not set.

Tried kernel org 3.17-rc7 as it was reported in above as working, but not for
me.