Clear fGpu in GrVkResourceProvider::PipelineStateCache::release

Clear the borrowed fGpu backpointer in PipelineStateCache::release()
during GPU teardown to avoid holding a dangling pointer.

This will avoids a `DanglingUntriaged` pointer after the MiraclePtr
rewrite.

Bug: 351867706
Doc: https://bit.ly/miracleptr-skia
Change-Id: Iad408f4e4df44192caf6576cbd0b781f5bdb75f0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1340477
Reviewed-by: Greg Daniel <egdaniel@google.com>
Auto-Submit: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Greg Daniel <egdaniel@google.com>