Vulkan: Fix access mask of generateMipmap's blit Bug: angleproject:8143 Bug: angleproject:7125 Change-Id: I6e5b6cd1f445c2c41d4b78aeb368e30cc4c5354b Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4475444 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com>