powerpc/64: Set DSCR default initially from SPR

Take the DSCR value set by firmware as the dscr_default value,
rather than zero.

POWER9 recommends DSCR default to a non-zero value.

Signed-off-by: From: Nicholas Piggin <npiggin@gmail.com>
[mpe: Make record_spr_defaults() __init]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>