drm: Extract drm_dumb_buffers.c

Just code movement, doc cleanup will follow up later.

v2: Keep all the copyright notices.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114115825.22050-2-daniel.vetter@ffwll.ch