drm/kconfig: Remove unused select of DRM_KMS_FB_HELPER

The option DRM_KMS_FB_HELPER has been removed. Also remove the last
remaining select statement for it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 91185d55 ("drm: Remove DRM_KMS_FB_HELPER Kconfig option")
Acked-by: Maxime Ripard <maxime@cerno.tech>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210526100825.29450-1-tzimmermann@suse.de