drm/msm: dpu: Remove dpu_power_handle_get_dbus_name()

It's only used for debugfs, so just output the enum value instead.

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>