During cases where vblank callback registration happens during
CWB is enabled and deregistered at point of no CWB, WB encoder
is left with dangling vblank_cb. Added changes to avoid registering
vblank callback on the clone mode encoder.
Change-Id: I62aa12ef453166d2f5558852d924f87841872f37
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>