drm/exynos/fimc: fix source buffer registers

FIMC in default mode of operation uses only one input buffer,
but the driver used also second buffer, as a result only the
first frame was processed correctly. The patch fixes it.

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>