drm/exynos: pass the correct pipe number

Instead of giving -1 to as arg to  drm_send_vblank_event() pass the
correct pipe number to it.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>