drm: Add consistency check for page-flipping

Driver implementations of the drm_crtc's .page_flip() function are
required to update the crtc->fb field on success to reflect that the new
framebuffer is now in use. This is important to keep reference counting
on the framebuffers balanced.

While at it, document this requirement to keep others from falling into
the same trap.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>