Stop using GrBackendSurface's pixel config

Note: I'll follow this up with a separate CL that removes the pixel config

Bug: skia:6718
Change-Id: If069afa95bd51d5d6b24089fd3a8526e4d982820
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228257
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed