drm/msm/sde: bug fix for PA VLUT feature via LUT DMA

Make sure ctl is valid before enabling/disabling PA VLUT so that
PA VLUT flush bit will be set correctly.
This change also fixes couple of memory leak bugs in the PA VLUT
programing via LUT DMA path.

Change-Id: I0b90d404c1291a89a110cff97a5a989e18e5f916
Signed-off-by: Ping Li <pingli@codeaurora.org>