perf/amd/uncore: Make L3 thread mask code more readable

Convert the l3_thread_slice_mask() function to use the more readable
topology_* helper functions, more intuitive variable names like shift
and thread_mask, and BIT_ULL().

No functional changes.

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/20200313231024.17601-2-kim.phillips@amd.com