panfrost: Copy blend constant into variant even when reusing it

Otherwise future lookups will match searches for the old constant.

Fixes: bbff09b9521 ("panfrost: Move the blend shader cache at the device level")
Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/6355
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16343>
1 file changed