efi: avoid error message when booting under Xen

efifb_probe() will issue an error message in case the kernel is booted
as Xen dom0 from UEFI as EFI_MEMMAP won't be set in this case. Avoid
that message by calling efi_mem_desc_lookup() only if EFI_MEMMAP is set.

Fixes: 38ac0287 ("fbdev/efifb: Honour UEFI memory map attributes when mapping the FB")
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Juergen Gross <jgross@suse.com>