OMAPDSS: FEATURES: Create a range param to get max downscaling

Create a dss_range_param member called FEAT_PARAM_DOWNSCALE to get the maximum
downscaling possible on the current platform. Use this in
dispc_ovl_calc_scaling().

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>