Fix shadow fb allocation size (v2)

->bitsPerPixel is rather obviously eight times too large.

v2: Use ->displayWidth - the pitch - not ->virtualX (Keith Packard)
Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

src/fbdev.c

@@ -827,8 +827,8 @@ FBDevScreenInit(SCREEN_INIT_ARGS_DECL)
 	fPtr->fbstart = fPtr->fbmem + fPtr->fboff;
 
 	if (fPtr->shadowFB) {
-	    fPtr->shadow = calloc(1, pScrn->virtualX * pScrn->virtualY *
-				  pScrn->bitsPerPixel);
+	    fPtr->shadow = calloc(1, pScrn->displayWidth * pScrn->virtualY *
+				  ((pScrn->bitsPerPixel + 7) / 8));
 
 	    if (!fPtr->shadow) {
 		xf86DrvMsg(pScrn->scrnIndex, X_ERROR,