drm: fb_helper: improve CONFIG_FB dependency

My previous patch correctly addressed the possible link failure, but as
Jani points out, the dependency is now stricter than it needs to be.

Change it again, to allow DRM_FBDEV_EMULATION to be used when
DRM_KMS_HELPER and FB are both loadable modules and DRM is linked into
the kernel.

As a side-effect, the option is now only visible when at least one DRM
driver makes use of DRM_KMS_HELPER. This is better, because the option
has no effect otherwise.

Fixes: 606b1028

 ("drm: fb_helper: fix CONFIG_FB dependency")
Suggested-by: Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20211029120307.1407047-1-arnd@kernel.org