drm/exynos: add exynos_drm_crtc_init function

drm/exynos: add exynos_drm_crtc_init function

Since exynos_drm_crtc is a struct which maps 1:1 to underlying device it
is better to put it directly into device's context instead of allocating
it separately. Another benefit is possibility of initialisation of
its fields directly, without expanding exynos_drm_crtc_create which is
already overloaded with number of arguments.
As a first step of the transition initialisation function should be
created.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>