drm/amdgpu: null check for hmm_pfns ptr before freeing it

Due to low memory or when num of pages is too big to be
accomodated, allocation could fail for pfn's.

Chekc hmm_pfns for NULL before calling the kvfree for the it.

Signed-off-by: Sunil Khatri <[e-mail]>
Acked-by: Arunpravin Paneer Selvam <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/amdgpu/amdgpu_hmm.c

···
 	if (!range)
 		return;
 
-	kvfree(range->hmm_range.hmm_pfns);
+	if (range->hmm_range.hmm_pfns)
+		kvfree(range->hmm_range.hmm_pfns);
+
 	amdgpu_bo_unref(&range->bo);
 	kfree(range);
 }