drm/exynos/fimc: simplify buffer queuing

The patch removes redundant checks, redundant HW reads
and simplifies code.

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>