drm/exynos: added comments and code clean.

this patch adds the following comments and code clean.
- add comment of exynos_drm_crtc_apply() call at page flip time.
- add comment that when exynos_drm_fbdev_reinit() is called,
  why num_connector is 0 and also the framebuffers should be destroyed.
- remove buf_off member from struct exynos_drm_overlay because this member
  isn't used anymore.

Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>