drm/omapdrm: Remove struct drm_fb_helper from struct omap_fbdev.

Store instances of drm_fb_helper and struct omap_fbdev separately.
This will allow omapdrm to use the common fbdev client, which allocates
its own instance of struct drm_fb_helper.

There is at most one instance of each per DRM device, so both can be
referenced directly from the omap and DRM device structures. A later
patchset might rework the common fbdev client to allow for storing
both, drm_fb_helper and omap_fbdev, together in the same place.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240924071734.98201-80-tzimmermann@suse.de