Don't initialize XV if we can't reserve a scalable sunxi-disp layer

In the case if an attempt to reserve a scalable sunxi-disp layer
failed, don't initialize XV at all. Otherwise any attempt to use
XV overlay is not going to work correctly and just results in
the following dmesg spam:

[  728.280000] [DISP] not supported yuv channel format:18 in img_sw_para_to_reg

This may happen on Allwinner A13 if scaler mode is enabled in
.fex file (A13 only has one DEFE scaler). Allwinner A10 also
can have similar troubles in dual-head configuration if scaler
mode is enabled for one or both screens (A10 has two DEFE scalers).
Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>

src/sunxi_video.c

@@ -273,9 +273,16 @@ static XF86AttributeRec Attributes[] =
 SunxiVideo *SunxiVideo_Init(ScreenPtr pScreen)
 {
     ScrnInfoPtr pScrn = xf86Screens[pScreen->myNum];
+    sunxi_disp_t *disp = SUNXI_DISP(pScrn);
     SunxiVideo *self;
     XF86VideoAdaptorPtr adapt;
 
+    if (!disp || !disp->layer_has_scaler) {
+        xf86DrvMsg(pScreen->myNum, X_INFO,
+                   "SunxiVideo_Init: no scalable layer available for XV\n");
+        return NULL;
+    }
+
     if (!(self = calloc(1, sizeof(SunxiVideo)))) {
         xf86DrvMsg(pScreen->myNum, X_INFO, "SunxiVideo_Init: calloc failed\n");
         return NULL;