powerpc/perf: Drop the case of returning 0 as instruction pointer

Drop the case of returning 0 as instruction pointer since kernel
never executes at 0 and userspace almost never does either.

Fixes: e6878835 ("powerpc/perf: Sample only if SIAR-Valid bit is set in P7+")
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210818171556.36912-2-kjain@linux.ibm.com