powerpc/perf: Update default sdar_mode value for power9

Commit 20dd4c62 ('powerpc/perf: Fix SDAR_MODE value for continous
sampling on Power9') set the default sdar_mode value in MMCRA[SDAR_MODE]
to be used as 0b01 (Update on TLB miss). And this value is set if sdar_mode
from event is zero, or we are in continous sampling mode in power9 dd1.

But it is preferred to have the sdar_mode value for power9 as
0b10 (Update on dcache miss) for better sampling updates instead
of 0b01 (Update on TLB miss).

From Anton:

Using a bandwidth test case with a 1MB footprint, I profiled cycles and
chose TLB updates of the SDAR:

  $ perf record -d -e r000400000000001E:u ./bw2001 1M
                        ^
                        SDAR TLB

  $ perf report -D | grep PERF_RECORD_SAMPLE | sed 's/.*addr: //' | sort -u | wc -l
  4

  I get 4 unique addresses. If I ran with dcache misses:

  $ perf record -d -e r000800000000001E:u ./bw2001 1M
                        ^
                        SDAR dcache miss

  $ perf report -D|grep PERF_RECORD_SAMPLE| sed 's/.*addr: //'|sort -u | wc -l
  5217

I get 5217 unique addresses. No surprises here, but it does show why
TLB misses is the wrong event to default to - we get very little useful
information out of it.

Signed-off-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Acked-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>