drm: fb_helper: fix CONFIG_FB dependency

drm: fb_helper: fix CONFIG_FB dependency

With CONFIG_FB=m and CONFIG_DRM=y, we get a link error in the fb helper:

aarch64-linux-ld: drivers/gpu/drm/drm_fb_helper.o: in function `drm_fb_helper_alloc_fbi':
(.text+0x10cc): undefined reference to `framebuffer_alloc'

Tighten the dependency so it is only allowed in the case that DRM can
link against FB.

Fixes: f611b1e7624c ("drm: Avoid circular dependencies for CONFIG_FB")
Link: https://lore.kernel.org/all/20210721152211.2706171-1-arnd@kernel.org/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210927142816.2069269-1-arnd@kernel.org