Fixed issue with is_debug gn arg

Multiple is_debug arguements caused the script not to actually check the
is_debug arg from fuchsia.git. We now use JSON output to ensure we are
checking the is_debug arguement at the root.

Change-Id: I3e4b9ff1de26a05acf1ad352ca047378c1693fc2
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/github.com/intel/media-driver/+/688847
Reviewed-by: John Bauman <jbauman@google.com>