Move GrPixelConfig to GrTypesPriv

BUG= skia:6718

Change-Id: I254a5d289c1216e580b8f7fe613236c090d0e901
Reviewed-on: https://skia-review.googlesource.com/116196
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
6 files changed