drm/virtio: use drm_crtc_send_vblank_event()

Replace the legacy drm_send_vblank_event() with the new helper function.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465224105-21485-6-git-send-email-gustavo@padovan.org