drm/msm/dp: fix error handling conditions in lphw hpd

The sub-module initializations return an error rather
than a NULL pointer whenever there is a failure while
initialization. This change will update those
conditions to check for errors and NULL pointer.

Change-Id: I9713997176f8117dc8b1eb07e41f3dbce1c6f516
Signed-off-by: Sankeerth Billakanti <sbillaka@codeaurora.org>