drm/msm/sde: clear exclusion rect settings on lastclose

Clear the exclusion rect settings when the default value (NULL)
is passed in as the property value instead of a valid user
pointer.  This occurs when the last DRM client closes the driver.
Doing this ensures that any prior exclusion rect settings do not
get re-applied the next time the driver is opened.

Change-Id: Iefc994299eff526ead767e86b6794b761a2f336f
Signed-off-by: Steve Cohen <cohens@codeaurora.org>