drm/exynos: add helper to get crtc from pipe

The helper abstracts out conversion from pipeline
to crtc. Currently it is used in two places, but
there will be more uses in next patches.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>