msm: mdss: Move histogram completion initialization to pp_init

The histogram completion structures are initialized in pp_init, in
order to avoid the wait_queue initialization being called multiple times.
The init_completion in hist_enable has been replaced with a safer
function that does not modify the wait_queue.

Change-Id: If4dfc6a74069d9adc867131201e6f74c3c81c1ac
Signed-off-by: Benet Clark <benetc@codeaurora.org>