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

include/drm/drmP.h

@@ -80,6 +80,7 @@
 #include <drm/drm_file.h>
 #include <drm/drm_debugfs.h>
 #include <drm/drm_ioctl.h>
+#include <drm/drm_sysfs.h>
 
 struct module;
 
@@ -512,10 +513,6 @@ static inline int drm_device_is_unplugged(struct drm_device *dev)
  * DMA quiscent + idle. DMA quiescent usually requires the hardware lock.
  */
 
-			       /* sysfs support (drm_sysfs.c) */
-extern void drm_sysfs_hotplug_event(struct drm_device *dev);
-
-
 /*@}*/
 
 /* returns true if currently okay to sleep */