drm/amdgpu: fix unnecessary NULL check warnings

Remove NULL checks before vfree() to fix these warnings:
drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c:102:2-8: WARNING: NULL
check before some freeing functions is not needed.
Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -98,8 +98,7 @@ static int amdgpu_cs_bo_handles_chunk(struct amdgpu_cs_parser *p,
 	return 0;
 
 error_free:
-	if (info)
-		kvfree(info);
+	kvfree(info);
 
 	return r;
 }