Remove mutex in GrVkSemaphore

The mutex was originally added to support the use of multiple GrContext's
accessing the semaphore when using cross context images. However, we no
longer use semaphores for Vulkan cross contexts. We require the VkQueue to
be the same in the shared context, so this allows a simple barrier to do
the trick for us.

Change-Id: I92c80df6b14249de85e35c56c90a0910ad6c5bfc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/213300
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
3 files changed