msm_fb: display: release mdp clk in mdp4_overlay_writeback_on()

mdp clk is not released in mdp4_overlay_writeback_on()

CRs-fixed: 433986

Change-Id: Ida121e03b0f08ca312c60fb8d945fe2011df853f
Signed-off-by: Mayank Goyal <goyalm@codeaurora.org>