powerpc/irq64: Remove get_irq_happened()

No need to open code the read of local_paca->irq_happened in
assembly, we have READ_ONCE() for doing the same.

Replace get_irq_happened() by READ_ONCE(local_paca->irq_happened).

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/af511b53e4eb51f8fbc51eda7f5597175e68dce6.1652859593.git.christophe.leroy@csgroup.eu