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

commit 3104b812 upstream.

hw doesn't like a 0 value.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/radeon/atombios_encoders.c

@@ -332,6 +332,10 @@ 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 LVDS */
 	if (radeon_encoder->active_device & (ATOM_DEVICE_LCD_SUPPORT))
 		radeon_panel_mode_fixup(encoder, adjusted_mode);