hwc: dont return from vsync thread when read fails.

Read can return fail when the panel is off.
When read fails do not exit from the vsync thread,
continue with the vsync signal, as later there will
be correct vsync event from the driver

Change-Id: I0f145b93fe5d365eafb6b9e7661160f00f84db40
CRs-Fixed: 417529