drm/msm/sde: move to idle state before doze_suspend

Wait for last frame transfer complete and move to
idle state before entering in doze_suspend state.
This avoid clocks left ON during this power state
transition.

Change-Id: Ic50ed8e1d4cc422b70b66174db138a137ec1ff15
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>