coreboot_tables: specify clear interface for lb_framebuffer()

For some reason the "interface" for adding framebuffer information
is sitting in src/include/vbe.h while also guarding the call to
fill_lb_framebuffer() with vbe_mode_info_valid() along with some
macro if CONFIG_* for good measure.

Move the fill_lb_framebuffer() declaration to coreboot_tables.h and
provide a comment about how it should be used. Also, now that
there's no need for the notion of a global vbe_mode_info_valid()
remove it from the conditional call path of fill_lb_framebuffer().

Change-Id: Ib3ade6314624091ae70424664527a02b279d0c9b
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/19729
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

src/drivers/xgi/common/xgi_coreboot.c

@@ -426,9 +426,14 @@ int vbe_mode_info_valid(void)
 	return xgi_vbe_valid;
 }
 
-void fill_lb_framebuffer(struct lb_framebuffer *framebuffer)
+int fill_lb_framebuffer(struct lb_framebuffer *framebuffer)
 {
+	if (!vbe_mode_info_valid())
+		return -1;
+
 	*framebuffer = xgi_fb;
+
+	return 0;
 }
 
 struct xgifb_video_info *xgifb_video_info_ptr;