drm: Move atomic debugfs functions into drm_crtc_internal.h

This is not driver interface stuff.

Fixes: 6559c901 ("drm/atomic: add debugfs file to dump out atomic state")
Cc: Rob Clark <robdclark@gmail.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213230814.19598-3-daniel.vetter@ffwll.ch