drm/omap: Stop using drm_framebuffer_unregister_private

This is the deprecated function for when you embedded the framebuffer
somewhere else (which breaks refcounting). But omapdrm is using
drm_framebuffer_remove and a free-standing fb, so this is rendundant.

Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1482835765-12044-4-git-send-email-daniel.vetter@ffwll.ch