add --backend skp to fm

And move --backend first among the backend flags.

Change-Id: I853ede2a8218f046c968ace2ce8c5e277421e225
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/204133
Auto-Submit: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>