intel/fs: Set up sampler message headers in the visitor on gen7+

intel/fs: Set up sampler message headers in the visitor on gen7+

This gives the scheduler visibility into the headers which should
improve scheduling.  More importantly, however, it lets the scheduler
know that the header gets written.  As-is, the scheduler thinks that a
texture instruction only reads it's payload and is unaware that it may
write to the first register so it may reorder it with respect to a read
from that register.  This is causing issues in a couple of Dota 2 vertex
shaders.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104923
Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Francisco Jerez <currojerez@riseup.net>