Reset GrVkCommandBuffer's work status in end()

Since GrVkCommandBuffers are recycled, the persistant work status was disabling the skipping of empty flushes

Change-Id: Id551b967a8a73c07f802a393ba00053a0115e96b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/205834
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>