drm: Kill DRM_HZ

We don't have any userspace interfaces that use HZ as a time unit, so
having our own DRM define is useless.

Remove this remnant from the shared drm core days.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/exynos/exynos_mixer.c

@@ -868,7 +868,7 @@ static void mixer_wait_for_vblank(void *ctx)
 	 */
 	if (!wait_event_timeout(mixer_ctx->wait_vsync_queue,
 				!atomic_read(&mixer_ctx->wait_vsync_event),
-				DRM_HZ/20))
+				HZ/20))
 		DRM_DEBUG_KMS("vblank wait timed out.\n");
 }