drm/gem: Clear mapping addresses for unused framebuffer planes

Set the returned mapping address to NULL if a framebuffer plane does
not have a BO associated with it. Likewise, ignore mappings of NULL
during framebuffer unmap operations. Allows users of the functions to
perform unmap operations of certain BOs by themselfes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210730183511.20080-4-tzimmermann@suse.de