intel/fs: fixup sampler header message on Gfx11+

What does this MR do and why?

intel/fs: fixup sampler header message

If you look at the sampler message header Gfx9+, you'll see that we
mostly only use 2 dwords (dw2 & dw3). DW2 has a bunch of sampler
parameters, DW3 is the sampler handle.

On Gfx9 we can micro optimize by copying r0 into the header because
the HW mostly doesn't care. We just have to clear dw2 on non VS/FS
stages.

On Gfx11+, we always have to do a careful copy of the r0.3 bits to
mask out the bottom unrelated bits. So there, just clearing the entire
header makes more sense.

On Xe2+, the dw4 of the header references the sampler feedback surface
handle and bit0 is a boolean to know whether to use that surface or
not. So it *REALLY* matters to have that as 0. If we copy r0, we'll
get random bits in dw4, leading to enable that surface.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable

/cc @shadeslayer