drm/radeon: use drm_mode_vrefresh() rather than mode->vrefresh

commit 3d2d98ee1af0cf6eebfbd6bff4c17d3601ac1284 upstream.

Just in case it hasn't been calculated for the mode.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>