drm: Remove explicit vrefresh initialization from DRM_MODE()

No need to zero initialize .vrefresh in DRM_MODE() since it's using
desgignated initializers.

This will also avoid some duplicate initialization warnings later.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>