soc: qcom: pil: Refine error handling for SCM ICC client

A NULL return for ICC get is valid for platforms where
CONFIG_INTERCONNECT is not defined. Refine the error handling
for the scm perf client to allow a NULL value.

Change-Id: I714a9091000e99aecb466a54968a140868659410
Signed-off-by: Anant Goel <anantg@codeaurora.org>