hdmitx: enhance robustness for export function [1/1]

PD#SWPL-52333

Problem:
hdmitx audio setting function is called on TV
platform and cause crash

Solution:
if hdmitx driver is not enabled, export function
just return

Verify:
am301

Change-Id: I5e369e6951dfbb5da1784a7245b9c08bc7684c6f
Signed-off-by: hang cheng <hang.cheng@amlogic.com>