drm/exynos: remove struct exynos_drm_manager

exynos_drm_manager was just a redundant struct to represent the crtc as
well. In this commit we merge exynos_drm_manager into exynos_drm_crtc to
remove an unnecessary level of indirection easing the understand of the
flow on exynos.

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