drm/rockchip: use DRM_DEV_ERROR for log output

Replace the use of the dev_err macro with the DRM_DEV_ERROR
DRM helper macro. rockchip driver uses almost exclusively DRM_* logging
(aside from 2 dev_* instances, one of which is converted in this patch),
so this makes things more consistent.

Signed-off-by: Wambui Karuga <wambui.karugax@gmail.com>
[seanpaul expanded on the commit message]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191107092945.15513-1-wambui.karugax@gmail.com