[ALPS08729654] mt8792: disp: fix coverity issues of null pointer defect

[Description]
Fix coverity issues:ALPS08729654

[Test]
build pass

MTK-Commit-Id: 0b82f9b962d2294eeca4dcfc9ec1ee06bd8c005a

Signed-off-by:  <ot_jjun.zhang&lt;ot_jjun.zhang@mediatek.com>
CR-Id: ALPS08729654
Feature: display_mobile
Change-Id: I8712c7a75fcc04fbc36823b9917420d43807a990