[ALPS08716617] mt6878: disp: fix coverity issues of null pointer defect

[Description]
Fix coverity issues: ALPS08745115,  ALPS08745123.

[Test]
MEStress Pass.

MTK-Commit-Id: 0253ba1bfd4d956b17288f8c911106beef89d05a

Signed-off-by: kai.shi" <kai.shi@mediatek.com>
CR-Id: ALPS08716617
Feature: display_mobile
Change-Id: I007e41e9ede7f1f06f70a3c0733656ba03f576bb