drm/msm: drop pm ops from the headless msm driver

The msm_pm_prepare()/msm_pm_complete() only make sense for the
KMS-enabled devices, they have priv->kms guards inside. Drop global
msm_pm_ops, which were used only by the headless msm device.

Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/561643/


Signed-off-by: Rob Clark <robdclark@chromium.org>