modetest.c calculated refresh rate wrong for interlaced modes

at least with my equipment it ends up calculating the refresh rate as half of it's true value. Here's the fix:

static float mode_vrefresh(drmModeModeInfo *mode)
{
        return  mode->clock * (mode->flags & DRM_MODE_FLAG_INTERLACE ? 2000.00 : 1000.00)
                        / (mode->htotal * mode->vtotal);
}