drm/msm/sde: clear dim-layer settings when setting default value

Clear all dim-layers when the default value of 0 (NULL) is set for
the dim layer property.  This setting needs to be cleared when
resetting the crtc custom properties during lastclose.

Change-Id: Ib451f918d7448e4952d086827852516445f137c1
Signed-off-by: Steve Cohen <cohens@codeaurora.org>