powerpc: Rearrange and fix show_interrupts() for sparse irq_descs

Move the default case out of the if, ie. when we're just displaying
an irq. And consolidate all the odd cases at the top, ie. printing
the header and footer.

And in the process cope with sparse irq_descs.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>