drm/omap: remove omap_drm_win

struct omap_drm_window is only used to pass plane setup data to
omap_framebuffer_update_scanout(). This can as well be accomplished by
just passing the DRM state.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>