perf/amd/uncore: Prepare L3 thread mask code for Family 19h

In order to better accommodate the upcoming Family 19h, given
the 80-char line limit, move the existing code into a new
l3_thread_slice_mask() function.

No functional changes.

 [ bp: Touchups. ]

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200313231024.17601-1-kim.phillips@amd.com