msm: mdss: Reorder code to avoid null pointer dereference

Set mdss_res to NULL after mdss_mdp_pp_term function in err
case inside mdss_mdp_probe function. This change can avoid
null pointer dereference if err occurs during mdss_mdp_probe.

Change-Id: I90b289e03f2711d45dcb80d9fa6eeb78b447cfd4
Signed-off-by: Ping Li <quicpingli@codeaurora.org>