intel/vec4: Don't try both sources as immediates for DPH

DPH isn't actually commutative, so this doesn't work.  If the immediate
in src0 would be a VF candidate, we could do better. *shrug*

No shader-db changes on any Intel platform.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Fixes: b04beaf4 ("intel/vec4: Try both sources as candidates for being immediates")
(cherry picked from commit 92252219)