fbdev: Remove conflict-handling code

Remove the call to do_remove_conflicting_framebuffers() from the
framebuffer registration. Aperture helpers take care of removing
conflicting devices. With all ownership information stored in the
aperture datastrcutures, remove remove_conflicting_framebuffers()
entirely.

This change also rectifies DRM generic-framebuffer registration, which
tried to unregister conflicting framebuffers, even though it's entirely
build on top of DRM.

v2:
	* remove internal aperture-overlap helpers, which are
	  now unused

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220718072322.8927-12-tzimmermann@suse.de