drm/exynos: expose struct exynos_drm_crtc

Let other pieces of the driver access struct exynos_drm_crtc as well.

struct exynos_drm_manager will be merged into struct exynos_drm_crtc, in
the sense we will move all its members to exynos_drm_crtc, so to start
this conversion exynos_drm_crtc need to be exposed as well.

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