i965: Convert VUE map generation checks to if rather than switch.

i965: Convert VUE map generation checks to if rather than switch.

There are currently only two VUE map layouts: one for Gen4-5, and one
for everything else.  We keep having to add new "case N+1" labels for
every new hardware generation, and so far it's always been the same.

This patch makes it so we only have to do work in the case where
something actually changes.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>