drm/exynos: fix build error caused by removed drm core macros

DRM_HZ and DRM_WAKEUP macros are removed, but they still remained.
So this patch fixes build errors by replacing the macros.

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