gpu: nvgpu: Set pd_cache to NULL after it's freed

Clear the pointer to the non-existent pd_cache after the
pd_cache is freed. This is good practice and makes it easier
to verify that this function has accomplished its intended
purpose in the unit tests.

JIRA NVGPU-1323

Change-Id: I1e1c20344a385bc96b293f2007485e7f4d99f947
Signed-off-by: Alex Waterman <alexw@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/1941535
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Reviewed-by: svc-misra-checker <svc-misra-checker@nvidia.com>
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Scott Long <scottl@nvidia.com>
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drivers/gpu/nvgpu/common/mm/pd_cache.c

@@ -215,6 +215,7 @@ void nvgpu_pd_cache_fini(struct gk20a *g)
 	}
 
 	nvgpu_kfree(g, g->mm.pd_cache);
+	g->mm.pd_cache = NULL;
 }
 
 /*