drm: fb_helper: improve CONFIG_FB dependency

drm: fb_helper: improve CONFIG_FB dependency

[ Upstream commit 9d6366e743f37d36ef69347924ead7bcc596076e ]

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: 606b102876e3 ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>