mdss: ppp: free ppp bw when not in use for some time

For every request ppp was toggling on and off bw and clock.
This behavior was not for ideal bw driver and would also increase
the total time to process ppp request. Now ppp request for bw and
keeps it until there has been no additional requests for over
50ms.

Change-Id: I09e27d2ecba5ed7f9af4fef2f3722bfa90967944
Signed-off-by: Terence Hampson <thampson@codeaurora.org>