powerpc/rtas: Use rtas_call_unlocked() in call_rtas_display_status()

Although call_rtas_display_status() does actually want to use the
regular RTAS locking, it doesn't want the extra logic that is in
rtas_call(), so currently it open codes the logic.

Instead we can use rtas_call_unlocked(), after taking the RTAS lock.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>