drm: omapdrm: Switch to the universal plane API

Remove the CRTC private planes by switching to the universal plane API.
This results in a merge of the CRTC private plane created by the driver
(omap_crtc->plane) and the CRTC primary plane created by the DRM core
(crtc->primary).

Reference counting of the framebuffers in the update plane operation is
thus simplified as no reference needs to be stored in the private plane
anymore.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>