vgacon: rework screen_info #ifdef checks

On non-x86 architectures, the screen_info variable is generally only
used for the VGA console where supported, and in some cases the EFI
framebuffer or vga16fb.

Now that we have a definite list of which architectures actually use it
for what, use consistent #ifdef checks so the global variable is only
defined when it is actually used on those architectures.

Loongarch and riscv have no support for vgacon or vga16fb, but
they support EFI firmware, so only that needs to be checked, and the
initialization can be removed because that is handled by EFI.
IA64 has both vgacon and EFI, though EFI apparently never uses
a framebuffer here.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Khalid Aziz <khalid@gonehiking.org>
Acked-by: Helge Deller <deller@gmx.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20231009211845.3136536-3-arnd@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/alpha/kernel/setup.c

@@ -131,6 +131,7 @@ static void determine_cpu_caches (unsigned int);
 
 static char __initdata command_line[COMMAND_LINE_SIZE];
 
+#ifdef CONFIG_VGA_CONSOLE
 /*
  * The format of "screen_info" is strange, and due to early
  * i386-setup code. This is just enough to make the console
@@ -147,6 +148,7 @@ struct screen_info screen_info = {
 };
 
 EXPORT_SYMBOL(screen_info);
+#endif
 
 /*
  * The direct map I/O window, if any.  This should be the same