x86/PCI: Eliminate remove_e820_regions() common subexpressions

Add local variables to reduce repetition later.  No functional change
intended.

Link: https://lore.kernel.org/r/20220304035110.988712-2-helgaas@kernel.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>