drm/exynos: kill exynos_drm_crtc::pipe

Since crtc index is stored in drm_crtc pipe field became redundant.
The patch beside removing the field simplifies also
exynos_drm_crtc_get_pipe_from_type.

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