drm/msm/sde: Disable 3d lut through reg dma

When using reg dma hardware block, currently feature was enabled using
reg dma where as it was being disabled via ahb path. In Reg dma
implementation all writes should go through reg dma block.

Change-Id: Idc572c291d6516afcfcf0c1b2cfa708f9ba2be95
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>