gallium/ttn: Fix setup of outputs_written.
We need a 64-bit value, otherwise we only handle the low 32, and happen to sign-extend to claim to write all varying slots if VARYING_SLOT_VAR2 was used.
We need a 64-bit value, otherwise we only handle the low 32, and happen to sign-extend to claim to write all varying slots if VARYING_SLOT_VAR2 was used.