hwc2: Restore null display mode

Skip SDM Core API calls in null display mode. Fix null-pointer
dereference while booting up in null display mode. This is a
follow-up change to Change-Id Ia274017...

CRs-Fixed: 2356380
Change-Id: I5b8c63cf0f13df6763f1fa078a61109f784e0270
1 file changed