drm/msm/sde: fix invalid loop-condition in clear dim_layer

Fix the loop-condition to clear dim_layer settings in all
the stages including the max supported stage.

Change-Id: I0d5637eac8f14973adac443b2383f2b5aa7fa587
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>