i965: Implement rasterizer discard via SOL unless required for queries.

We currently use CL_INVOCATION_COUNT for the GL_PRIMITIVES_GENERATED
query, which involves passing all primitives to the clipper.  When
rasterizer discard is enabled, we program the clipper in REJECT_ALL
mode, rather than using the SOL stage's "Rendering Disable" feature.

See commit f09b91f78247409f54c975f56cb10d5f350fe64e for an explanation
of why we implement GL_PRIMITIVES_GENERATED this way.

Apparently the SOL stage's "Rendering Disable" feature is a lot faster
than having the clipper reject all primitives.  It's safe to use when
no GL_PRIMITIVES_GENERATED query is active, as we don't care about
CL_INVOCATION_COUNT incrementing.

This patch makes us use SO_RENDERING_DISABLE when no query is active,
but continues falling back to the clipper in REJECT_ALL mode when the
queries are enabled.  It brings back the perf_debug for the clipper
case (which I removed in commit 1f9445ff57b, thinking it wasn't useful).

Improves performance in Gl32GSCloth by 84.8303% +/- 2.07132% (n = 10)
on my Broadwell GT2 laptop.

Cc: [email protected]
Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Jason Ekstrand <[email protected]>

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