i965: Use 3DSTATE_CLIP's User Clip Distance Enable bitmask on Gen8+.

Gen6-7.5 specify the user clip distance enable bitmask in 3DSTATE_CLIP.
Gen8+ normally uses the new internal signalling mechanism to select the
one specified in the last enabled shader stage (3DSTATE_VS, DS, or GS).

This is a pretty good fit for Vulkan, or even newer GL, where the
bitmask comes entirely from the shader.  But with glClipPlane(),
this is dynamic state, and we have to listen to _NEW_TRASNFORM.

Clip plane enables are the only reason the VS/DS/GS atoms need to
listen to _NEW_TRANSFORM.  3DSTATE_CLIP already has to listen to it
in order to support ARB_clip_control settings.

Setting the "Use the 3DSTATE_CLIP bitmask" force enable bit allows
us to drop _NEW_TRANSFORM from all the shader stage atoms, so we can
re-emit them less often.

Improves performance of OglBatch7 (version 6) by 2.70773% +/- 0.491257%
(n = 38) at 1024x768 on Cherryview.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

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