drm/exynos/fimd: stop using manager->ctx pointer

The patch replaces accesses to manager->ctx pointer by container_of
construct. As fimd was the last user of ctx the patch removes
this field as well.

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