drm/exynos: Add an initialize function to manager and display

This patch adds an initialize function to the manager and display
operations. This allows them to keep track of drm_device in their
local context, as well as adds an initialization hook right after
the encoder is created.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>