x86: use old screen_info if needed

With modern drm/kms graphic driver kexec-tools does not setup screen_info
correctly so one will only see screen output after those drm drivers
reinitializing after rebooting. Copying the old screen info from original
boot_params will help during my test, although it could not work for some
potential cases, but it is not worse than before.  This has been used in
the kernel kexec_file_load.

Signed-off-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Simon Horman <horms@verge.net.au>