i965/skl: Send a message header when doing constant loads SIMD4x2

i965/skl: Send a message header when doing constant loads SIMD4x2

Commit 0ac4c272755c7 made it add a header for the send message when
using SIMD4x2 on Skylake because without this it will end up using
SIMD8D. However the patch missed the case when a sampler is being used
to implement constant loads from a buffer surface in a SIMD4x2 vertex
shader.

This fixes 29 Piglit tests, mostly related to the ARL instruction in
vertex programs.

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Tested-by: Anuj Phogat <anuj.phogat@gmail.com>