drm: Remove drm_mode_config::fb_base

The fb_base in struct drm_mode_config has been unused for a long time.
Some drivers set it and some don't leading to a very confusing state
where the variable can't be relied upon, because there's no indication
as to which driver sets it and which doesn't.

The only usage of fb_base is internal to two drivers so instead of trying
to force it into all the drivers to get it into a coherent state
completely remove it.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Thomas Zimmermann <tzimemrmann@suse.de>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221019024401.394617-1-zack@kde.org