drm: Extract drm_debugfs.h

Doc polish will follow in the next patch.

v2: Put the include guard #endif at the end (Ville).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170322205336.24549-1-daniel.vetter@ffwll.ch