msm: display: Proper handling of NULL pointer access for MDDI sysfs node.

All A-family chips which don't include MDDI interface module
observe NULL pointer access error while trying to access MDDI
debugfs nodes. This change fixes the issue with proper handling
of NULL pointer access.

Change-Id: Ie5be96bb2b9c28b256ab3999079c2c7cd9bc54a8
Signed-off-by: Sangani Suryanarayana Raju <csuryan@codeaurora.org>