disp: msm: sde: update uidle wd timer load value and fal1 threshold

Update the uidle wd timer load value to 18. This change will allow
a 15us wd timer per hardware recommendation.

Update fal1 threshold value to take the minimum of 15 or the
current setting which takes line time and target idle time into
consideration. The target idle time is also being updated from 10us to
40us.

Change-Id: Ia8d9c2070813beef18fdf342526d82cf8f82989b
Signed-off-by: Samantha Tran <samtran@codeaurora.org>