drm/exynos: hdmi: replace fb size with mode size from win commit

For default graphic window, mixer_win_commit() sets display size
register as fb size. Calling setplane with smaller fb size than
mode size to default window causes distorted display result. So
this patch replaces fb size with mode size for display size from
the mixer_win_commit().

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>