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/include/vbe.h

@@ -111,7 +111,6 @@ typedef struct {
 #define VESA_GET_MODE_INFO	0x4f01
 #define VESA_SET_MODE		0x4f02
 
-int vbe_mode_info_valid(void);
 void vbe_set_graphics(void);
 void vbe_textmode_console(void);