drm/exynos: fimd: fix broken dp_clock control

Commit 1feafd3a ("drm/exynos: add
exynos5420 support for fimd") add support for Exynos 5420 SoC, but it
broke enabling display clock feature because of incorrect condition
check. This patch fixes it, so display is working again on platforms
requiring display clock control (i.e. Exynos5250-based SNOW platform).

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>