iris: Enable Gen11 Color/Z write merging optimization

TCCNTLREG contains additional L3 cache write merging optimizations.

The default value on my system appears to be:
- URB Partial Write Merging (bit 0)
- L3 Data Partial Write Merging (bit 2)
- TC Disable (bit 3)

Windows drivers appear to set bit 1 as well to enable "Color/Z Partial
Write Merging".  This should solve an issue we were seeing where MRT
benchmarks were using substantially more bandwidth than they ought.
However, we have not observed it to cause measurable FPS gains.

It is unclear whether we should be setting bit 0 or bit 3, so for now
we leave those at the hardware default value.

Improves performance in Manhattan 3.0 by 6% on ICL 8x8 at a fixed
frequency, according to Felix Degrood.  I didn't see any improvements
at out-of-the-box power management settings, however.

Acked-by: Jason Ekstrand <[email protected]>

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