disp: msm: sde: fix imbalance power vote during cwb and dms concurrency

Imbalance power vote caused with below scenario:

1. Primary display running with CWB enabled.
2. Idle-PC has kicked in causing primary and cwb encoder to rc_idle state.
3. Post idle-PC with DMS switch enabled along with CWB.

Changes made are to avoid pre_modeset and post_modeset callbacks
for CWB encoder as it not required and it puts a unnecessary power
vote causing power issues in suspend usecase.

Change-Id: I95b7ba093484f50be5f575cafa63a02e8fbca441
Signed-off-by: Jayaprakash Madisetty <jmadiset@codeaurora.org>