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: I84d3f91ccb8cd1ee013677e2c05fcf24ae20db21
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>