i965: Cite the 965 PRM for "the data cache is the sampler cache".

i965: Cite the 965 PRM for "the data cache is the sampler cache".

Presumably, this comment exists to justify the usage of
I915_GEM_DOMAIN_SAMPLER for this relocation.  At one point, this was
necessary to ensure that the right flushing was done to keep caches
coherent.  These days, the kernel just flushes everything, so I don't
think it matters.

Still, the comment is interesting, so leave it in place.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>