drm/exynos: decon5433: Fix WINCONx reset value

The only bits that should be preserved in decon_win_set_fmt() is
WINCONx_ENWIN_F. All other bits depends on the selected pixel formats and
are set by the mentioned function.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>