powerpc/watchpoints: Simplify watchpoint reinsertion

We only remove watchpoints when they have the perf_single_step flag set,
so we can reinsert them during the first iteration.

Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230801011744.153973-5-bgray@linux.ibm.com