liboverlay: Correctly roundoff the Downscale factor

Rotator needs to be engaged for downscale factor
1.3 and above to prevent excessive load on mdp
resulting in underruns while playing 1080p Video
on 720p panels

Change-Id: Ie35d7f90ece080e8f318118974d62f86852df64b
CRs-Fixed: 620313