drm/msm: add pointer check before dereference

Add pointer check, print error message and return
error value in case the pointer is NULL. Also this change
uses connector iterator to go through the list of connectors
in the mode_config.

Change-Id: I9b776bfce771148606548a41cab6a695fc2adfa7
Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>
Signed-off-by: Narendra Muppalla <NarendraM@codeaurora.org>