drm/exynos: fimd: fix window clear code

To correctly disable hardware window during driver init, both enable bits
(WINCONx_ENWIN in WINCON and SHADOWCON_CHx_ENABLE in SHADOWCON) must be
cleared, otherwise hardware fails to re-enable such window later.

While touching this function, also temporarily disable ctx->suspended flag
to let fimd_wait_for_vblank function really to do its job.

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