i965: Shrink Gen5 VUE map layout to be the same as Gen4.

i965: Shrink Gen5 VUE map layout to be the same as Gen4.

The PRM suggests a larger layout, mostly to support having
gl_ClipDistance[] somewhere predictable for the fixed-function clipper
-- but it didn't actually arrive in Gen5.

Just use the same layout for both Gen4 and Gen5.

No Piglit regressions.

Improves performance in CS:S Video Stress Test by ~3%.

V2: - Remove now-useless function for determining the SF URB read offset
    - Remove now-unused BRW_VARYING_SLOT_POS_DUPLICATE

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>