i965: Cite the Ivybridge PRM for SFID enum values.

i965: Cite the Ivybridge PRM for SFID enum values.

The Ivybridge PRM adds new SFIDs and lists them in a different volume
than Sandybridge, so it's worth adding a reference.

I also removed the BSpec reference, as the section it referred to
was moved somewhere, and I couldn't find it.  This leaves one Haswell
SFID without a citation, but we can add one once the PRMs are out.

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