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

BugLink: http://bugs.launchpad.net/bugs/1580754

commit 0126d4b9 upstream.

hw doesn't like a 0 value.
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/gpu/drm/amd/amdgpu/atombios_encoders.c

@@ -298,6 +298,10 @@ bool amdgpu_atombios_encoder_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 (amdgpu_encoder->active_device & (ATOM_DEVICE_LCD_SUPPORT))
 		amdgpu_panel_mode_fixup(encoder, adjusted_mode);