remove prepForSampling from GrSurfaceContext::writePixels.

We believe with advent of  GrBackendSurfaceMutableState that this is
only needed for inline atlas uploads. Those call GrGpu directly.

Bug: 1191058
Change-Id: Iad851ebc9f5b7df21d16064a84caa9575da98634
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/393082
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
8 files changed