Unbind xfer buffer in a couple more places

Move code into a helper function, make sure that all TexImage and
TexSubImage calls are correctly unbinding.

Bug: skia:
Change-Id: I79bfa0af43a08f775186a867167fa67b4437688c
Reviewed-on: https://skia-review.googlesource.com/42640
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed