drm/exynos: fimd: calculate the correct address offset

Calculate the correct address offset values for alpha and color key
control registers based on exynos4 and exynos5 user manuals.
Also remove VIDOSD_C_SIZE_W0 macro and fix comments about registers for
size and alpha.

Signed-off-by: Leela Krishna Amudala <l.krishna@samsung.com>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>