disp: msm: sde: Secondary display lm preference with lm pair

When secondary display preference is determined and 2 layer
mixers are required, the preference must check LM pair mask
to meet hw restrictions.

Change-Id: I22845be84f95659a58be98ff11afa4e652fb16e3
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>