Vulkan: Clear mCurrentGraphicsPipeline after createPipelineLayout()
Vulkan: Clear mCurrentGraphicsPipeline after createPipelineLayout()

Set mCurrentGraphicsPipeline = nullptr when the pipeline layout is
recreated if immutable samplers are used to prevent a use-after-free.

For a fuller discussion on why this is necessary, see:
Vulkan: clear mCurrentGraphicsPipeline during invalidate |
https://chromium-review.googlesource.com/c/angle/angle/+/2647746

Bug: b/178424566
Bug: angleproject:5624
Change-Id: I70679bda85cd587dbd8893a4576ee5d33aee2b70
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2683041
Commit-Queue: Tim Van Patten <c7814369a8d3d5671c484acc7b3215f1bf4221be@google.com>
Reviewed-by: Shahbaz Youssefi <582eeec4f08a2ccd61e3ef52115e6e7c571582de@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle