drm/fb-helper: document remove*_conflicting_framebuffers()

Copy remove*_conflicting_framebuffers() kerneldocs from fbdev code
to make DRM developers' life easier.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/f70c1cc4a4f77dd9bad58fc7ca344609c0a91fa7.1536342228.git.mirq-linux@rere.qmqm.pl