i965/skl: Force the exec size to 8 when initing header for SIMD4x2

i965/skl: Force the exec size to 8 when initing header for SIMD4x2

On Gen9+ there needs to be a header when sampling using SIMD4x2. The
header is set up by copying from the g0 register. Commit 07c571a39f
tried to fix this mov instruction to always use an exec size of 8
because previously it was incorrectly using 4. It did this by casting
the type of the destination register to vec8. This was done because
there is code in brw_set_dest to guess the exec size based on the
width of the dest register. However I misunderstood how this works
because it is actually only used when the width is less than 8. That
means the patch actually changed it to use the default exec size which
on SIMD16 would be 16 and the MOV would clobber over the first
register in the send message. This patch makes it additionally set the
default exec size to 8. This is similar to how the message is set up
in fs_generator::generate_tex.

I think this wasn't picked up by any Piglit tests because we don't
have any fragment shaders that hit this code path so nothing was using
SIMD16. However the patch caused failures in deqp tests.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90153
Reviewed-by: Matt Turner <mattst88@gmail.com>
Tested-by: Tapani Pälli <tapani.palli@intel.com>