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 ee4484be3dc827cf15bcf109f5
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 <[email protected]>
Reported-by: Eero Tamminen <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Reviewed-by: Kristian Høgsberg <[email protected]>
Cc: [email protected]

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