i965: (trivial) rename computes stencil to gen9

i965: (trivial) rename computes stencil to gen9

All the documentation I can find says that this bit (and functionality) only
exists on SKL+. Since the bit isn't yet used, there is no real impact here.

The original code was added by Ken here (a surprisingly long time ago):
commit f3c6d6f1e151f6a44a76038dccebe4434038dcb1
Author: Kenneth Graunke <kenneth@whitecape.org>
Date:   Thu Nov 29 21:00:27 2012 -0800

    i965: Update 3DSTATE_PS, 3DSTATE_WM, and add 3DSTATE_PS_EXTRA.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>