drm/radeon: make sure vertical front porch is at least 1

hw doesn't like a 0 value.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/atombios_encoders.c

@@ -310,6 +310,10 @@ static bool radeon_atom_mode_fixup(struct drm_encoder *encoder,
 	    && (mode->crtc_vsync_start < (mode->crtc_vdisplay + 2)))
 		adjusted_mode->crtc_vsync_start = adjusted_mode->crtc_vdisplay + 2;
 
+	/* vertical FP must be at least 1 */
+	if (mode->crtc_vsync_start == mode->crtc_vdisplay)
+		adjusted_mode->crtc_vsync_start++;
+
 	/* get the native mode for scaling */
 	if (radeon_encoder->active_device & (ATOM_DEVICE_LCD_SUPPORT)) {
 		radeon_panel_mode_fixup(encoder, adjusted_mode);