disp: msm: sde: dynamic lm reservation for secondary disp

Primary and secondary displays should have first priority
when reserving lms. Static reservation can potentially block
higher resolutions for the required displays. This patch gets
the layer mixer requirement for primary or secondary display
if available. It reserves those layer mixers dynamically
for the respective display when connector is registered.

Change-Id: Id69dac4c72d6b20008049f4aeb71c0f97d0a426b
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>