drm/msm/sde: Fix uninitialized variables and pointers

Return values in some cases can be wrongs as its not
initialized properly. Perform proper NULL check for
pointers as well.

Change-Id: Ieac1a8f50dabd2918edaba1bd93599a62b8c9bf0
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>