i965: Handle rasterizer discard in the clipper rather than SOL on Gen7.

i965: Handle rasterizer discard in the clipper rather than SOL on Gen7.

In order to implement the GL_PRIMITIVES_GENERATED query in a sane
fashion on our hardware, we can't discard primitives until the clipper.
The patch after next explains the rationale.

By setting the clipper to REJECT_ALL mode, all primitives get thrown away,
so rendering is still appropriately disabled.

This may negatively impact performance in the rasterizer discard case,
but it's unclear how much and this hasn't been observed to be a
bottleneck in any application we've looked at.  The clipper is the very
next stage in the pipeline, so I don't think it will be terrible.

v2: Add a perf_debug; resolve rebase conflicts on the brw dirty flags.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Paul Berry <stereotype441@gmail.com>