disp: msm: sde: handle LTM switch in and out of dual pipe merge mode

When LTM is switching on/off, merge_mode bit value gets toggled between
0x1 (dual pipe merge configuration) and 0x0 (single pipe configuration).
It is illegal to reconfigure LTM to/from dual-pipe merge mode before
both LTM instances have completed their current workloads. This change
adds support to disable merge_mode one frame after histogram is disabled
to make sure both hardware instances are completely idle and avoid
corrupted histogram data collection.

Change-Id: I9a6b5cbfb69e8af7936749e57fe7c8f7c2703b95
Signed-off-by: Ping Li <pingli@codeaurora.org>