drm/fb-helper: Add TODO for making drm_fb_helper_alloc_fbi fill apertures

Currently drivers using drm_fbdev_generic_setup() end up with a single
empty aperture in their fb_info struct.

Not having the proper info in the apertures list causes
register_framebuffer to not remove conflicting framebuffers,
which some drivers currently workaround by manually calling
drm_fb_helper_remove_conflicting_pci_framebuffers().

Add a TODO as a reminder that we need to fix this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200326151009.102377-1-hdegoede@redhat.com