drm: gm12u320: Use DRM_DEV_ERROR everywhere

Previously the driver was using a mix of DRM_ERROR and dev_err, be
consisent and use DRM_DEV_ERROR everywhere instead.

Cc: Sam Ravnborg <sam@ravnborg.org>
Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190730133857.30778-2-hdegoede@redhat.com