drm: Don't export dp-aux devnode functions

They're only used internally within the dp helpers. Also nuke the
kerneldoc (we only document the driver interface in the drm shared
functions). And move the header file from the public include/
directory to the source files into drm_crtc_helper_internal.h, similar
to how we already have drm_crtc_internal.h.

While at it also move drm_fb_helper_modinit since that belongs in
there, too.

I noticed this all since I spotted kerneldoc which wasn't pulled into
the rst templates.

v2: Update Copyright date.

Cc: Sean Paul <seanpaul@chromium.org>
Cc: Rafael Antognolli <rafael.antognolli@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/1471034937-651-16-git-send-email-daniel.vetter@ffwll.ch