By letting TextureVk::syncState know it's being called for
generateMipmap, it can make a better decision to initialize the image:

- Staged updates to mips that are going to be overwritten are dropped
- The image is created with full mipchain to avoid a redefine in the
  following generateMipmap() call.

Bug: angleproject:4551
Change-Id: Ic70ee6c0a0b29c7bd62beaff612b2f2d5276defb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2249340
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>