drm: don't let crtc_ww_class leak out

kbuild spotted this error, with drm/msm patches that add a new
modeset-lock in the driver and driver built as a module:

  ERROR: "crtc_ww_class" [drivers/gpu/drm/msm/msm.ko] undefined!

Really the only reason for crtc_ww_class not being internal to
drm_modeset_lock.c is that drm_modeset_lock_init() was static-inline
(for no particularly good reason).

Fix that, and move crtc_ww_class into drm_modeset_lock.c.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1479163257-18703-1-git-send-email-robdclark@gmail.com