OMAPDSS: DISPC: Clean up scaling related clock and five tap calculations

Move DISPC scaling related code from dispc_ovl_setup() to a new function
dispc_ovl_calc_scaling(). Use overlay caps to check if the overlay can scale or
not. No functional changes are made.

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