drm/exynos: cleanup device pointer usages

Struct device pointer got from platform device pointer is already
alsigned as variable, but some functions do not use device pointer.
So this patch replaces thoes usages.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>