drm/omap: remove dispc_ops

dispc_ops was created to help with the multi-module architecture and
giving us the possibility of multiple dispc implementations. Neither of
these is valid anymore, and we can remove dispc_ops and use direct
calls to dispc.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201215104657.802264-54-tomi.valkeinen@ti.com