x86/efi: Cleanup efi_enter_virtual_mode() function

Add two small functions:
efi_merge_regions() and efi_map_regions(), efi_enter_virtual_mode()
calls them instead of embedding two long for loop.

Signed-off-by: Dave Young <dyoung@redhat.com>
Acked-by: Borislav Petkov <bp@suse.de>
Tested-by: Toshi Kani <toshi.kani@hp.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>