disp: msm: sde: fix min pre-downscale check

De-couple the horizontal and vertical pre-downscale checks since
this block can be used in cases where only X-axis downscaling
is needed.

Change-Id: I2e7d30863baed98e9f7fa0a328837691f0bc75a5
Signed-off-by: Steve Cohen <cohens@codeaurora.org>
Signed-off-by: Jayaprakash <jmadiset@codeaurora.org>