drm/exynos: modify the compatible string for exynos fimd

drm/exynos: modify the compatible string for exynos fimd

modified compatible string for exynos4 fimd as "exynos4210-fimd" and
exynos5 fimd as "exynos5250-fimd" to stick to the rule that compatible
value should be named after first specific SoC model in which this
particular IP version was included as discussed at
https://patchwork.kernel.org/patch/2144861/

Signed-off-by: Vikas Sajjan <vikas.sajjan@linaro.org>
Acked-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>