drm/exynos: fimd: fix dma burst size setting for small plane size

This patch fixes trashed display of buffers cropped to very small width.
Even if DMA is unstable and causes tearing when changing the burst size,
it is still better than displaying a garbage.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>