graphics/mesa-dri: Always disable intel-rt on !amd64

Since anv is only included for x86(-64) the MESON_ON isn't added to MESON_ARGS.
So explicitely disable intel-rt on !amd64 since it doesn't work.

Reported by: eduardo@freebsd.org
Spoonsored by: Beckhoff Automation GmbH & Co. KG