i965/skl: Always use a header for SIMD4x2 sampler messages

i965/skl: Always use a header for SIMD4x2 sampler messages

SKL+ overloads the SIMD4x2 SIMD mode to mean either SIMD8D or SIMD4x2
depending on bit 22 in the message header.  If the bit is 0 or there is
no header we get SIMD8D.  We always wand SIMD4x2 in vec4 and for fs pull
constants, so use a message header in those cases and set bit 22 there.

Based on an initial patch from Ken.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>