drm/msm: free atomic state during lastclose

Ensure that the refcount of the atomic state is decremented
during lastclose. This will make sure that when the atomic
state is freed, all objects being held by it are released.

Change-Id: I547cf997b9eb80560c9646f688071fb6a41648b2
Signed-off-by: Fuad Hossain <fhossain@codeaurora.org>