drm/nouveau: Do not include <drm/drm_fb_helper.h>

Including <drm/drm_fb_helper.h> is not required, so remove the include
statement. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20230111130206.29974-9-tzimmermann@suse.de