i965 gen6: Implement rasterizer discard.

This patch enables rasterizer discard functionality (a part of
transform feedback) in Gen6, by generating an alternate GS program
when rasterizer discard is active.  Instead of forwarding vertices
down the pipeline, the alternate GS program uses a URB Write message
to deallocate the URB entry that was allocated by FF sync and
terminate the thread.

Note: parts of the Sandy Bridge PRM seem to imply that we could do
this more efficiently, by clearing the GEN6_GS_RENDERING_ENABLE bit,
and not allocating a URB entry at all.  However, it's not clear how we
are supposed to terminate the thread if we do that.  Volume 2 part 1,
section 4.5.4, says "GS threads must terminate by sending a URB_WRITE
message with the EOT and Complete bits set.", and my experiments so
far confirm that.

Reviewed-by: Kenneth Graunke <[email protected]>

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