drm/msm: dpu: Remove dpu_power_client

There's only one client -- core, and it's only used for runtime pm which
is already refcounted.

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>