drm/exynos: Fix FIMD buffer size calculation

Commit adacb228 ("drm: Exynos: Respect framebuffer pitch for
FIMD/Mixer") fixed the buffer size calculation by using the FB
pitch value but later commit 26b9c2813ede1 ("drm/exynos: remove
struct *_win_data abstraction on planes") added a regression so
fix the buffer size calculation again.

Tested on Chromebook Snow / Peach Pit.

Fixes: 26b9c2813ede1 ("drm/exynos: remove struct *_win_data abstraction on planes")
Signed-off-by: Daniel Stone <daniels@collabora.com>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Inki Dae <inki.dae@samsung.com>