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

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

Signed-off-by: Alex Deucher <[e-mail]>
Cc: [e-mail]

drivers/gpu/drm/radeon/r600_dpm.c

···
 		list_for_each_entry(crtc, &dev->mode_config.crtc_list, head) {
 			radeon_crtc = to_radeon_crtc(crtc);
 			if (crtc->enabled && radeon_crtc->enabled && radeon_crtc->hw_mode.clock) {
-				vrefresh = radeon_crtc->hw_mode.vrefresh;
+				vrefresh = drm_mode_vrefresh(&radeon_crtc->hw_mode);
 				break;
 			}
 		}