powerpc/perf/hv_gpci: Probe failures use pr_debug(), and padding reduced

fixup for "powerpc/perf: Add support for the hv gpci (get performance
counter info) interface".

Makes the "not enabled" message less awful (and hidden unless
debugging).

Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>