msm: sde: disp: Set merge_mode after vlut and hist enable

To ensure that set merge_mode after vlut and hist enable, move merge_mode
enable from ltm_init to ltm_vlut.
Redefine the OP_MASK of LTM_INIT_ENABLE and LTM_VLUT_ENABLE, in order to
write the merge_mode bits correctly.

Change-Id: I5258e7f545e265b114098e46d31986274127e962
Signed-off-by: Ping Li <pingli@codeaurora.org>
Signed-off-by: Yuchao Ma <yuchaom@codeaurora.org>