vbe: remove the necessity for a global vbe_mode_info_valid()

There's no users of vbe_mode_info_valid() aside from the local
compilation units. Remove the declaration and make the function
static to the current global implementers.

Change-Id: I4872ac6ad15ba6a86bba69d51a8348b9921c152d
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/19730
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/device/oprom/realmode/x86.c

@@ -216,7 +216,7 @@ static void setup_realmode_idt(void)
 vbe_mode_info_t mode_info;
 static int mode_info_valid;
 
-int vbe_mode_info_valid(void)
+static int vbe_mode_info_valid(void)
 {
 	return mode_info_valid;
 }

src/device/oprom/yabel/vbe.c

@@ -156,7 +156,7 @@ vbe_info(vbe_info_t * info)
 
 static int mode_info_valid;
 
-int vbe_mode_info_valid(void)
+static int vbe_mode_info_valid(void)
 {
 	return mode_info_valid;
 }