disp: msm: sde: fix max downscale check for legacy HW

Legacy HW that does not support pre-downscale capability will
incorrectly fail certain scaling checks. Fix those checks to
fully support HW without pre-downscaler.

Change-Id: I8f645bbff959e176c1d4d05a30a580113e320d4b
Signed-off-by: Steve Cohen <cohens@codeaurora.org>