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

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

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20230111130206.29974-11-tzimmermann@suse.de