drm/exynos: make sure that hardware overlay for fimd is disabled

Changelog v2:
wait for VSYNC instead of BACKPORCH.

Changelog v1:
the values set to registers will be updated into real registers
at vsync so dma operation could be malfunctioned when accessed
to memory after gem buffer was released. this patch makes sure
that hw overlay is disabled before the gem buffer is released.

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