drm: omapdrm: Don't setup planes manually from CRTC .enable()/.disable()

Planes setup is handled by the DRM core through the atomic helpers,
there's no need to duplicate the code in the CRTC .enable() and
.disable() operations.

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