disp: msm: sde: properly reset LTM merge_mode bit and portrait_en bit

Reset the LTM merge_mode bit and portrait_en bit if LTM histogram and
VLUT are both disabled.

Change-Id: Ica9f681e458afe8c7cd1dfe8ba718cf46eadac3b
Signed-off-by: Ping Li <pingli@codeaurora.org>
Signed-off-by: fengjuan <fengjuan@codeaurora.org>