powerpc/32s: fix booting with CONFIG_PPC_EARLY_DEBUG_BOOTX

When booting through OF, setup_disp_bat() does nothing because
disp_BAT are not set. By change, it used to work because BOOTX
buffer is mapped 1:1 at address 0x81000000 by the bootloader, and
btext_setup_display() sets virt addr same as phys addr.

But since commit 215b8237

 ("powerpc/32s: set up an early static
hash table for KASAN."), a temporary page table overrides the
bootloader mapping.

This 0x81000000 is also problematic with the newly implemented
Kernel Userspace Access Protection (KUAP) because it is within user
address space.

This patch fixes those issues by properly setting disp_BAT through
a call to btext_prepare_BAT(), allowing setup_disp_bat() to
properly setup BAT3 for early bootx screen buffer access.

Reported-by: Mathieu Malaterre <malat@debian.org>
Fixes: 215b8237

 ("powerpc/32s: set up an early static hash table for KASAN.")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Tested-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>