Skip disabled CRTCs in amdgpu_scanout_(do_)update

The vblank / page flip ioctls don't work as expected for a disabled CRTC.

(ported from radeon commit acc11877423ecd81a6e0a7f38466f80e43efee20)
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

src/amdgpu_kms.c

@@ -257,7 +257,8 @@ amdgpu_scanout_do_update(xf86CrtcPtr xf86_crtc, int scanout_id)
 	GCPtr gc;
 	BoxRec extents;
 
-	if (drmmode_crtc->dpms_mode != DPMSModeOn ||
+	if (!xf86_crtc->enabled ||
+	    drmmode_crtc->dpms_mode != DPMSModeOn ||
 	    !drmmode_crtc->scanout[scanout_id].pixmap)
 		return FALSE;
 
@@ -323,7 +324,8 @@ amdgpu_scanout_update(xf86CrtcPtr xf86_crtc)
 	DrawablePtr pDraw;
 	BoxRec extents;
 
-	if (drmmode_crtc->scanout_update_pending ||
+	if (!xf86_crtc->enabled ||
+	    drmmode_crtc->scanout_update_pending ||
 	    !drmmode_crtc->scanout[0].pixmap ||
 	    drmmode_crtc->dpms_mode != DPMSModeOn)
 		return;