drm: Consolidate and document sysfs support

- remove docs for internal func, doesn't add value
- add short overview snippet instead explaining that drivers don't
  have to bother themselves with reg/unreg concerns
- drop the ttm comment about drmP.h, drmP.h is disappearing ...

Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170404095304.17599-2-daniel.vetter@ffwll.ch