drm/msm/sde: trigger null commit if a display stuck in cont-splash

SDE driver marks the respective DRM display active when continuous
splash is enabled for that display. It expects a valid commit for
continuous splash enabled display before pm_suspend/pm_resume call.
It may possible that DRM driver client does not push any valid commit
on this display and it may try to enter pm_suspend with active
configuration. That breaks the pm resume commit because suspend state
cached continuous splash enabled commit. This change triggers a null commit
during pm_suspend if a display is stuck in continuous splash enabled state.

Change-Id: I99f978e08ef200a3a7e3d50d30665cfcc1596e2d
Signed-off-by: Prabhanjan Kandula <pkandula@codeaurora.org>