disp: msm: sde: avoid registering vblank callback during CWB

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>