drm/exynos: remove pipe member of struct exynos_drm_manager

It is not longer used. This is part of the process of removing
struct exynos_drm_manager entirely.

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