mdss: mdp: fix incorrect programming of scalar enable

In use-cases where scaling is required in one direction
and not on other, scalar extension parameter will be set
to update the scale register. Add proper check to this enable
scale bit only in the required direction.

Change-Id: I5527f22c83fe456d0124af12516c5bdb240e80a9
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>