drm/exynos: fix fb offset calculation for plane

There is no any reason to change fb offset when CRTC is out of screen.
Also, this fixes a typing error.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>