SK_API in src/gpu cleanup

Change-Id: I14d6f7ef06c3cfcefd0813dd5013e4185b12d8cc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/235804
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
15 files changed