powerpc/udbg: Make it safe to call udbg_printf() always

Make udbg_printf() check if udbg_putc is set, and if not just return.
This makes it safe to call udbg_printf() anytime, even when a udbg
backend has not been registered, which means we can avoid some ifdefs
at call sites.

Signed-off-by: Qian Cai <cai@lca.pw>
[mpe: Split out of larger patch, write change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>