sdm: Fix multiple VBlank registration

Currently VBlank registration happens even if polling thread wakes up
for other events. This causes multiple wake-ups on actual VBlank.
This change registers for VBlank in VBlank handler and on VBlank
enable, if it's not already registered.

Change-Id: I4aada5a5bd28382d60c68865c20eaabda5325ccf
CRs-fixed: 2167257
2 files changed