drm/msm: add wrapper for PM runtime when not using mdp5

Added a wrapper for runtime suspend/resume functions so that the
compilation of msm_drv does not break when not using an MDP5 based
MSM platform.

Change-Id: I4667a6d825aba7084446a02b284a6f903e5c6b13
Signed-off-by: Steve Cohen <cohens@codeaurora.org>