disp: msm: sde: separate horz/vert max downscale checks

Separate the horizontal and vertical max downscale checks
as pre-downscale introduced different limits on different
axes. Also cleanup the variable names for max downscale
limit when pre-downscale is not enabled.

Change-Id: If01aac1844d0bd5133502a50dbc38197e11da5d5
Signed-off-by: Steve Cohen <cohens@codeaurora.org>
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>