i965: Handle rasterizer discard in the clipper rather than GS on Gen6.

i965: Handle rasterizer discard in the clipper rather than GS on Gen6.

This has more of a negative impact than the previous patch, as on Gen6
passing primitives through to the clipper means we actually have to make
the GS thread write them to the URB.

I don't see another good solution though, and rasterizer discard is not
the most common of cases, so hopefully it won't be too terrible.

v2: Add a perf_debug; resolve rebase conflicts on the brw dirty flags;
    remove the rasterizer_discard field from brw_gs_prog_key.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net> [v1]
Reviewed-by: Paul Berry <stereotype441@gmail.com>