In makeLinearGamma, return the original CS if it's already linear

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2589

Change-Id: I51d8f8f64bc5f73a0b7cbf5f01f3d23b56c1895e
Reviewed-on: https://skia-review.googlesource.com/2589
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
1 file changed