i965: Remove the emit_linterp() helper.

i965: Remove the emit_linterp() helper.

Rather than computing the barycentric mode each time we emit a LINTERP,
we can simply compute it once, as soon as we know we're doing non-flat
interpolation.

At that point, emit_linterp() doesn't do much, so fold it into the
call sites and drop it.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>