drm/omap: Remove the obsolete #define omap_plane _omap_plane hack

Remove the obsolete "#define omap_plane _omap_plane" hack and other
related hacks to get around the enum omap_plane colliding with struct
omap_plane.

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