ppc64: Rearrange btext initialization for consistency with ppc32

Moved init_boot_display from arch/ppc64/kernel/pmac_setup.c to
arch/ppc64/kernel/btext.c and declared it in asm-ppc64/btext.h.
Call it from init_early rather than pmac_init_early.

Signed-off-by: Paul Mackerras <paulus@samba.org>