drm/exynos: Disable plane when released

this patch ensures that each plane connected to encoder is disabled
when released, by adding disable callback function of encoder helper

we had faced with one issue that invalid memory is accessed by dma
once drm is released and then the dma is turned on again. actually,
in our case, page fault was incurred with iommu. the reason is that
a gem buffer accessed by the dma is also released once drm is released.

so this patch would fix this issue ensuring the dma is disabled
when released.

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