bifrost: Fix packing of ADD_FEXP2_FAST

This was being packed as 1-src and so the Src1 was not set up properly.
It only worked by accident.

Signed-off-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6111>
2 files changed