intel: Apply Geminilake "Barrier Mode" workaround.

Apparently, Geminilake requires you to whack a chicken bit to select
either compute or tessellation mode for barriers.  The recommendation
is to switch between them at PIPELINE_SELECT time.

We may not need to do this all the time, but I don't know that it hurts
either.  PIPELINE_SELECT is already a pretty giant stall.

This appears to fix hangs in tessellation control shaders with barriers
on Geminilake.  Note that this requires a corresponding kernel change,

    drm/i915: Whitelist SLICE_COMMON_ECO_CHICKEN1 on Geminilake.

in order for the register write to actually happen.  Without an updated
kernel, this register write will be noop'd and the fix will not work.

Reviewed-by: Rafael Antognolli <[email protected]>

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