drm/msm: dpu: Move DPU_POWER_HANDLE_DBUS_ID to core_perf

It's only used in core_perf, so stick it there (and change the name to
reflect that).

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>