drm: omapdrm: inline omap_plane_setup into update/disable

At the moment we have omap_plane_setup() function which handles both
enabling (and configuring) and disabling the plane. With atomic
modesetting we have separate hooks for plane enable/config and disable.

This patch moves the code from omap_plane_setup() to
omap_plane_atomic_update() and omap_plane_atomic_disable().

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