mdss: mdp3: request and release clocks for every mdp operation

In the command mode panel, all the clocks can be voted off if
there is no display update.  To support this feature, every
operation which requires h/w access will be guarded with clock
vote on and off.

Change-Id: If56a656b9965a6044c38de5aa85969256d581121
Signed-off-by: Xiaoming Zhou <zhoux@codeaurora.org>