i965: Add a better PRM citation for the IMS dimension mangling.

i965: Add a better PRM citation for the IMS dimension mangling.

Paul originally had to reverse engineer these formulas based on the
description about how the sampler works.  The description here is not
the easiest to follow - especially given that it's from the Sandybridge
era, when the hardware only did 4x multisampling.

Jordan and I recently found another part of the documentation where they
simply state that IMS dimensions must be adjusted by a set of formulas.
Quoting this section provides an easy to follow explanation for the
code, including 2x/4x/8x/16x.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@intel.com>