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/drivers/xgi/common/xgi_coreboot.c

@@ -421,7 +421,7 @@ int xgifb_modeset(struct pci_dev *pdev, struct xgifb_video_info *xgifb_info)
 	return 0;
 }
 
-int vbe_mode_info_valid(void)
+static int vbe_mode_info_valid(void)
 {
 	return xgi_vbe_valid;
 }