x86/cpu: Rename and move CPU model entry for Diamond Rapids

This model was added as INTEL_PANTHERCOVE_X (based on the name of the
core) with a comment that the platform name is Diamond Rapids. It was
also placed at the end of the file in a new section for family 19
processors.

This is different from previous naming as Andrew Cooper noted.
PeterZ agreed and posted a patch[1] to fix the name and move it in
sequence with other Xeon servers. But without a commit description or
sign-off the patch wasn't ever applied.

Patch updated to cover one additional use of the #define by turbostat
and to change the "Family 6" comment to also list 18 and 19 since new
models in these families are mixed in with family 6.

Originally-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Sohil Mehta <sohil.mehta@intel.com>
Link: https://lore.kernel.org/all/20250214130205.GK14028@noisy.programming.kicks-ass.net/ # [1]