LoadIdentityClut: Add new case for some AMD cards on Linux.

Mesa 8 reports, e.g., “AMD CYPRESS”, for HD-5000, a case we don’t yet
handle.
By chance, the fallback path chose the proper gfxhwtype = 0 anyway. But
get rid of warning clutter.

-> A better approach on Linux (possibly OSX) would be to use DCE
display engine generation as a matching criterion instead -> More
“constant”.