Currently, the functions below take the omap_plane parameter and derive the
overlay caps within them. Pass the overlay caps as a parameter to the function
to allow these to be used by writeback too.

- dispc_ovl_set_zorder()
- dispc_ovl_set_pre_mult_alpha()
- dispc_ovl_setup_global_alpha()
- dispc_ovl_calc_scaling()
- dispc_ovl_setup()

These functions will be used for writeback later, and the caps will help in
deciding if they are to be used for writeback or not. This allows reuse of
overlay caps for writeback.

Using omap_overlay_caps for writeback seems a bit incorrect, but caps is
something already in use by users of OMAPDSS(omapfb/omap_vout), so we use
overlay caps for overlay like features of writeback too.
Signed-off-by: Archit Taneja <archit@ti.com>