If the buffer format of the PBO does not contain both depth and stencil,
it already has packed data ready for upload, so this change allows the
GPU path to be taken.

Bug: angleproject:5315
Bug: b/172354898
Change-Id: I424c15951594f49ffc140f9cde9e7c73486512b3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2526947Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>