meson: fix power8 option

Do not throw a deprecation warning if the power8 option is set to the
new 'disabled' value. Instead, warn if it is still set to the legacy
value 'false'.

Fixes: 138c003d2273 ("meson: deprecated 'true' and 'false' in combo options for 'enabled' and 'disabled'")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6370>
diff --git a/meson.build b/meson.build
index 76f72a9..bd6a1fc 100644
--- a/meson.build
+++ b/meson.build
@@ -765,7 +765,7 @@
 if _power8 == 'true'
   _power8 = 'enabled'
   warning('power8 option "true" deprecated, please use "enabled" instead.')
-elif _power8 == 'disabled'
+elif _power8 == 'false'
   _power8 = 'disabled'
   warning('power8 option "false" deprecated, please use "disabled" instead.')
 endif