drm: Fix typo in drm_event_reserve_init() kerneldoc

drm_event_reserve_init_locked() is the correct function to call when
already holding the dev->event_lock lock.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161215113602.27966-1-thierry.reding@gmail.com