drm/omapdrm: Remove fbdev from struct omap_drm_private

The DRM device stores a pointer to the fbdev helper. Remove struct
omap_drm_private.fbdev, which contains the same value. No functional
changes.

v2:
	* don't clear dev->fb_helper unnecessarily (Tomi)
	* include omap_fbdev.h in omap_fbdev.c (kernel test robot)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230403104035.15288-5-tzimmermann@suse.de