disp: msm: avoid removing BW vote when layers are staged

In cases like doze suspend bandwidth values are received
zero even though layers are staged. In such case previous
bandwidth vote is retained to avoid removal of votes from
display side.

Change-Id: I6b84f11c716c0509e8970cf3c2955ce5f9f9dfb0
Signed-off-by: Yashwanth <yvulapu@codeaurora.org>