i965/Gen4-5: ensure VUE slots for clipdistance are valid if user clipping is enabled.

i965/Gen4-5: ensure VUE slots for clipdistance are valid if user clipping is enabled.

V2: We don't particularly care where they fall in the VUE map, as long
as they are allocated somewhere, and occupy two contiguous slots. Don't
fiddle with the SF layout at all -- there's no need.

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