x86_64: there's no need to preallocate level1_fixmap_pgt

Early fixmap will allocate its own L1 pagetable page for fixmap
mappings, so there's no need to preallocate one.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>