drm/doc: clarify how to acquire required vblank event reference

As a driver write it is not entirely obvious that a reference to
the event e mentioned in the doc can be obtained via
drm_crtc_vblank_get(). Clarify how to obtain the reference.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180807201143.7446-1-stefan@agner.ch