Vulkan: Move ImageLayout define to earlier in vk_helpers.h file This is the preparation for the later CL. Later in the CL I need to define something like "angle::PackedEnumMap<ImageLayout, RefCountedEvent>" to use in CommandBufferHelperCommon. So I need ImageLayout to be defined earlier. No actual changes involved other than moving the code around for easier code review. Bug: b/336844257 Change-Id: I0a2553ef3aebf4b00d1798e2117fe6ed32ac172d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5484675 Commit-Queue: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>