drm: omapdrm: Remove omap_plane enabled field

The field tracks the plane state to avoid double-enable or -disable.
This isn't required anymore, as

- the DRM atomic core guarantees that the plane atomic_update and
  atomic_disable functions will never be called on an enabled/disabled
  plane

- the CRTC enable/disable operations that enable/disable the plane are
  already guarded against double enable/disable

We can thus remove the enabled field completely. The
omap_plane_set_enable() function then becomes a wrapper around
omap_plane_setup() which can be called directly.

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