msm: mdss: release mdp bandwidth voting for command mode when idle

There is no bandwidth usage for command mode panel if no new commit
comes, so mdp bandwidth request can be zero as soon as data transfer
to panel is done. This could bring power savings for use cases of
short transfer time relatively to long idle time (duty cycle) such as
partial update, and slow fps. When video mode panel is present, this
bandwidth cannot be released since the timing of this release is
unpredictable.

Change-Id: Ia67143aa421b7b8d8b1f359244dd397dab369b4a
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>