drm/msm/sde: break out plane decimation to separate properties

Separate the horizontal and vertical decimation configuration to
separate properties instead of bundling them with the scaler
property. This allows for some planes to support decimation even
if the customized scaler configuration is not available.

Change-Id: Icdb16d41139f2a90ffb4dc8a6fa73de5336f892a
Signed-off-by: Clarence Ip <cip@codeaurora.org>