i965/skl: Add a message header for the TXF_MCS instruction in vec4vs

i965/skl: Add a message header for the TXF_MCS instruction in vec4vs

When using SIMD4x2 on Skylake, the sampler instructions need a message
header to select the correct mode. This was added for most sample
instructions in 0ac4c2727 but the TXF_MCS instruction is emitted
separately and it was missed.

This fixes a bunch of Piglit tests which test texelFetch in a geometry
shader, for example:

 spec/arb_texture_multisample/texelfetch/2-gs-sampler2dms

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>