drm/msm: update rsc_min_threshold for sde rsc hw block

This change updates min_threshold to be large enough
for mode 0 execution. This threshold is the sum of
backoff time plus overhead from RSC/RSC_WRAPPER. The
overhead time required must be greater than 14. This
change uses the value 18.

Change-Id: I20923658b82af23595c35e07118fc089a427e1a7
Signed-off-by: Samantha Tran <samtran@codeaurora.org>