i965: Use BDW_MOCS_PTE for renderbuffers.

Write-back caching cannot be used for buffers being scanned out by the
display engine; surfaces used for scan-out must be write-through or
uncached.  I originally chose WT for render targets because it works in
all cases.  However, we really want to use write-back caching where
possible, as it is more efficient.

Most renderbuffers are not used for scanout - off-screen FBOs certainly
are fine, and non-pageflipped backbuffers should be fine as well.  So
in most cases WB will work.  However, we don't know what will be used
for scan-out, so we instead simply use the PTE value specified by the
kernel, as it knows these things.

This matches our MOCS choice on Haswell.

Fixes performance regressions since commit ee4484be3d
in a microbenchmark (spotted by Eero Tamminen).  Improves performance
in GLBenchmark 2.7/EgyptHD by 7.44362% +/- 0.496939% (n=55) on a
Broadwell GT2.  Improves performance in a bunch of other microbenchmarks
by ~15% or so.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reported-by: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Cc: mesa-stable@lists.freedesktop.org
(cherry picked from commit b7844d1248)

src/mesa/drivers/dri/i965/gen8_surface_state.c

@@ -377,7 +377,7 @@ gen8_update_renderbuffer_surface(struct brw_context *brw,
              horizontal_alignment(mt) |
              surface_tiling_mode(tiling);
 
-   surf[1] = SET_FIELD(BDW_MOCS_WT, GEN8_SURFACE_MOCS) | mt->qpitch >> 2;
+   surf[1] = SET_FIELD(BDW_MOCS_PTE, GEN8_SURFACE_MOCS) | mt->qpitch >> 2;
 
    surf[2] = SET_FIELD(width - 1, GEN7_SURFACE_WIDTH) |
              SET_FIELD(height - 1, GEN7_SURFACE_HEIGHT);