drm/exynos: use define instead of default_win member in struct mixer_context

The default_win member in struct mixer_context isn't change its value
after initialized to 0, so it's better using to define.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>