panfrost: Fix OOB array access compiler warning.

-1 is used to select "choose the dest type", but we were dereffing src[-1]
anyway and the compiler is not a fan.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6462>
1 file changed