drm/msm: dpu: Remove dpu_power_handle

Now that we don't have any event handlers, remove dpu_power_handle!

Changes in v2:
- None

Reviewed-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>