fb: change rules for global rules match.

Having a : should be enough 'fb:' isn't really useful
if the fb wants to a kms output ID.
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/video/fbmem.c

@@ -1789,7 +1789,7 @@ static int __init video_setup(char *options)
  		global = 1;
  	}
 
- 	if (!global && !strstr(options, "fb:")) {
+ 	if (!global && !strchr(options, ':')) {
  		fb_mode_option = options;
  		global = 1;
  	}