Since a6242e4d ("Vulkan: Support submitting multiple semaphores"),
we can submit up to 2 semaphores at a time, but pWaitDstStageMask
is still a single value. Change it to an array of two elements to
prevent an out of bounds access.

Bug: angleproject:3289

Change-Id: I5147802ce350af7b78dbf54cfa4a9519dd495f01
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1666347
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>