i965/vs: Fix Gen4/5 VUE map inconsistency with gl_ClipVertex

This is roughly a backport of Eric's commit 0967c362.

We avoided assigning a slot in the VUE map for gl_ClipVertex, but left
the bit set in outputs_written, producing horrible confusion further
down the pipe.

Mostly fixes rendering in source games, and probably in Freespace 2 SCP.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>

V2: Mask out the bit, not its index. Strangely, the game still worked
with that wrong, but rendering of pretty much anything else was
completely trashed.
i965/fs: Don't try to use bogus interpolation modes pre-Gen6.

Interpolation modes other than perspective-barycentric-pixel-center (and
their associated coefficients in the WM payload) only exist in Gen6 and
later.

Unfortunately, if a varying was declared as `centroid`, we would blindly
read the nonexistant values, and so produce all manner of bad behavior
-- texture swimming, snow, etc.

Fixes rendering in Counter-Strike Source and Team Fortress 2 on
Ironlake.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>