drm/exynos: dsi: make use of array for clock access

This patch make the driver to use an array for clock access. The number
of clocks are different from the existing MIPI DSI driver and Exynos5433
MIPI DSI driver. So this patch is needed before adding support for
Exynos5433 MIPI DSI driver.

Signed-off-by: Hyungwon Hwang <human.hwang@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>