drm/radeon: flush read cache for gtt with fence on r6xx and newer GPU V3

Cayman seems to be particularly sensitive to read cache returning
old data after bind/unbind to GTT. Flush read cache for GTT range
with each fences for all new hw. Should fix several rendering glitches.
Like

V2 flush whole address space
V3 also flush shader read cache

https://bugs.freedesktop.org/show_bug.cgi?id=40221
https://bugs.freedesktop.org/show_bug.cgi?id=38022
https://bugzilla.redhat.com/show_bug.cgi?id=738790



Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>