ENGR00171949:gpu-viv: change from mutex to completion for waiting pm events

ENGR00171949:gpu-viv: change from mutex to completion for waiting pm events

viv code is using mutex to wait for pm events,
kernel will see this as a dead lock and give a warrning, as the mutex
can be hold for a long time.

Signed-off-by: Wu Guoxing <b39297@freescale.com>
Acked-by: Lily Zhang