overlay: Add tolerance to downscale factor

In certain corner cases for 1080p panels, round doesn't get us
the right downscale factor resulting in MDP attempting to go into
blt mode. Add a tolerance for failure.

Bug: 9322550
Change-Id: I2be4715955d9e248fc63e408dc4a2cdf493152f6
Signed-off-by: Iliyan Malchev <malchev@google.com>