powerpc/book3e-64: Wait til generic_calibrate_decr to enable decrementer

Match what we do on 32-bit Book-E processors and enable the decrementer
in generic_calibrate_decr.  We need to make sure we disable the
decrementer early in boot since we currently use lazy (soft) interrupt
on 64-bit Book-E and possible get a decrementer exception before we
are ready for it.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>