drm: Consolidate dumb buffer docs

Put the callback docs into struct drm_driver, and the small overview
into a DOC comment.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114115825.22050-7-daniel.vetter@ffwll.ch