drm/framebuffer: Add framebuffer debugfs file

Add debugfs file that dumps info about the framebuffers and its planes.
Also dump info about any connected gem object(s).

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171107191348.17555-5-noralf@tronnes.org